In a multicat household, it's common to see one cat licking another. When some people see this behavior, known as allogrooming ("allo" means "another individual"), they assume it's a sign of friendship.

But is that really the case? Are there hidden, more complex reasons for why cats lick each other?

While research into allogrooming is ongoing, scientists have suggested different explanations for why cats lick each other, including some social reasons and some more practical ones.

Social bonding or social tension?

The historical view, based on observations of free-roaming-cat colonies, was that allogrooming is a sign of affection and helps cats create and maintain bonds with other cats in their social group, perhaps by creating a shared group smell.

But a 1998 study of 25 cats living together in an indoor-outdoor enclosure poked holes in this theory. The study found that aggressive behavior showed up in about 35% of allogrooming sessions, which suggested that allogrooming might not be about friendship. Instead, the authors proposed, it may help to defuse a tense situation in a confined space, where an actual fight would be too risky.

To investigate, Morgane Van Belle, a cat behavior scientist at Ghent University in Belgium, and her colleagues analyzed videos of allogrooming from 53 two-cat homes. They concluded that allogrooming may have different social functions, depending on the situation.

The team noticed that the cats sometimes licked each other while huddling together and copying one another's body postures. In these moments, allogrooming probably helps strengthen social bonds, the researchers proposed. Most licking was directed toward the ears; this may help cats relax because the skin there is rich in sensory nerve endings, Van Belle said. Allogrooming produces a relaxing effect in other animals too. For example, it lowers the heart rate in horses and triggers the release of feel-good hormones in monkeys.

They also noted that allogrooming sometimes led to friendly wrestling, in which the cats grabbed each other with their front legs and tumbled around. This suggests that allogrooming may serve as a signal to start play.

In other situations, however, the researchers noticed that allogrooming was followed by behaviors linked to conflict, such as turning the ears backward, pawing, and biting. In these cases, one cat would stand or lean over the other, and both cats showed subtle signs of stress like lip licking, head shaking or self-scratching.

Based on these observations, Van Belle's team think that allogrooming may be an "appeasement signal" in fraught moments that could lead to conflict or injury, for example, when one cat wants to take over a sleeping spot already occupied by another. They think it possible that the licking helps to calm things down and keeps a spat from turning into a real fight. Similar behavior has been seen in meerkats, where subordinate individuals groom dominant ones to placate them.

Yet because this licking often targets the neck — the same area cats bite during fights — it also may signal a subtle aggressive threat, the researchers suggested.

Hygiene and grooming

Van Belle and her colleagues think that, beyond sending social signals, allogrooming may help with hygiene. After all, it's often aimed at the head and neck ‪—‬ the areas that are "least accessible for the cat itself during grooming," Van Belle said. Allogrooming is thought to have a hygienic function in other species too. For example, ants achieve colony-level hygiene by grooming heavily infected individuals more, and primates concentrate their grooming on body parts the recipient can't easily reach. When it comes to cats, however, more research is needed. Van Belle's study did not measure parasites, infections or skin cleanliness to see whether such grooming actually reduces dirt or bugs.

Dr. Terry Curtis, a veterinary behaviorist who has studied how often allogrooming happens, also thinks cats might groom each other for hygiene reasons. She wonders whether feral cats, who likely carry more fleas than house cats, might have a different allogrooming pattern.

The explanations proposed by Van Belle and colleagues are reasonable but not definitive, Curtis said. Because so many factors can shape a cat's behavior, she's skeptical that researchers will ever be able to say with real confidence exactly why one cat grooms another.

After working as a veterinary behaviorist for 25 years, Curtis has concluded it is often impossible to understand what's going on in a cat's mind.

"Congratulations, you got a cat," she said. "Let the mystery begin.”

How much of a cat fan are you? Find out by taking our cat quiz!

Given that it's the closest planet to the sun, Mercury seems like it should be the hottest planet in our solar system.

However, at a blistering 900 degrees Fahrenheit (480 degrees Celsius), Venus tops Mercury's 800 F (430 C) highest surface temperature, despite being an average of 31 million miles (50 million kilometers) farther from the sun. So how can the second planet from our star be hotter than the closest planet to it?

It all comes down to reflectivity, atmospheric composition and geological history, experts told Live Science.

Totally different atmospheres

A planet's distance from its star is not the only factor that influences the planet's temperature.

"Distance tells us how much sunlight arrives at a planet, but it does not tell us how much is reflected … absorbed, how efficiently heat escapes, or how effectively the atmosphere transports heat around the planet," Stephen Kane, an astrophysicist who studies planetary habitability at the University of California, Riverside, told Live Science in an email. "Those properties can be just as important as distance, and sometimes much more important."

Mercury makes the case in miniature. According to Kane, the planet has essentially no atmosphere, so incoming sunlight strikes bare rock directly, heating it to extreme temperatures during the day. But with barely anything overhead to trap that warmth, Mercury radiates it straight back into space the moment the sun sets. As a result, nighttime temperatures plunge from roughly 800 F (430 C) during the day to about minus 290 F (minus 180 C) at night ‪—‬ a swing of well over 1,000 degrees, he added.

Venus tells the opposite story. Wrapped in an atmosphere that's roughly 90 times as dense as Earth's and consists almost entirely of carbon dioxide, Venus traps heat so effectively that its surface temperature barely changes at all, no matter where the sun happens to be, Kane explained.

A permanent blanket

Sunlight arrives at a planet mostly as near-infrared radiation and visible light, which pass through atmospheres consisting primarily of gases such as nitrogen, oxygen and carbon dioxide with relatively little trouble. Once the ground and lower atmosphere absorb that energy, though, they re-release it as infrared radiation, the kind of energy we feel as heat. Carbon dioxide happens to be excellent at grabbing and holding on to infrared radiation, Kane noted.

On Venus, the atmosphere is so deep and loaded with carbon dioxide that infrared energy leaving the surface gets absorbed and re-emitted over and over before any of it finally escapes into space. Some of that energy gets redirected back downward, Kane said, further warming the planet's lower atmosphere and surface. However, the heat isn't trapped forever, since energy conservation means that Venus must eventually release the same amount of energy it absorbs.

Notably, Venus doesn't actually soak up more sunlight than Mercury overall. Thick cloud cover reflects roughly three-quarters of incoming sunlight back into space before it ever reaches the ground, and only about 3% of the sunlight that arrives at Venus makes it down to the surface, Kane said. In fact, if that atmosphere and cloud deck were stripped away, a bare-rock Venus would actually run cooler than Mercury, since it would be receiving only about 29% as much sunlight from the start, he explained. It's the blanket, not the sunbathing, that makes Venus the hotter world.

Where the blanket came from

Venus' atmospheric blanket didn't spring into existence all at once, and how it formed is its own open question. Venus shows strong evidence of past and present volcanic and tectonic activity acting to smother the planet in greenhouse gases, Paul Byrne, a planetary scientist and associate professor of Earth, environmental and planetary sciences at Washington University in St. Louis, told Live Science in an email.

This volcanic activity has been so intense that Bryne describes modern Venus as sitting in a "post-runaway greenhouse" state, where its extreme heat has become a self-sustaining process regardless of what the planet's interior is doing at any given moment.

The heat isn't being generated from below; it's a consequence of the atmosphere Venus already has, locked in place by the same infrared-trapping effect described above.

Taken together, the experts' answers point to the same underlying lesson: How close a planet sits to its star is only the opening chapter of its climate story. What kind of atmosphere it has and how effectively that atmosphere holds on to heat are usually responsible for the rest.

See how well you know our planetary neighborhood with our solar system quiz!

For decades, engineers have been working to create smaller and smaller transistors to fit ever-larger numbers of them onto computer chips, all in the name of greater processing power. Now, in the age of artificial intelligence (AI), that effort has turned into an all-out race.

So how small can a transistor actually get? And is smaller even still the goal?

To understand the answer, first you need to know what a transistor is.

"If you take a computer or your mobile phone and you were to pry it open, you will see that there is a printed circuit board," Suman Datta, a professor of electrical and computer engineering at Georgia Tech, told Live Science.

On that circuit board are small, rectangular objects called chips, and inside each chip is a tiny piece of silicon. "And in that silicon, if you zoom in like a million times … maybe, if you're lucky, you will start seeing these tiny switches or transistors sitting etched into that piece of silicon," Datta said.

Each transistor is effectively a switch that turns on and off in response to jolts of electricity pulsing 4 billion times per second. Modern devices contain billions of transistors, and each one turning on and off in a precise orchestration to move and store 1s and 0s powers everything from Google searches and AI data centers to the device you're using to read these words.

The smaller the transistors are, the more can fit on each chip and the greater the chip's speed and functionality. So how small have they gotten so far?

"Researchers have built proof-of-concept devices in which a single atom controls the flow of electrons, although the rest of the device is still much larger," Anton Persson, an assistant professor at Chalmers University of Technology in Sweden, and Tara Peña, an incoming assistant professor at UCLA, told Live Science via a jointly written email. That means the switching part of a transistor has been made at the smallest scale physically possible, even though a complete device that small doesn't exist yet. New materials and approaches could make these devices even smaller. In a study published in June in the journal Nature Nanotechnology, Persson, Peña and colleagues used two-dimensional semiconductors made of tungsten disulfide, among other materials, to shrink nanoribbon transistors down to a channel width of 25 nanometers ‪—‬ about 0.00025 the width of a human hair.

Two-dimensional semiconductors are electrically active materials that are only one or a few atoms thick, and because electrical current can be controlled more precisely in such a thin layer, they allow for smaller transistors than traditional silicon does.

Over the next decade, they still expect silicon transistors to shrink, but more slowly than they did in the past, they said. "To go considerably smaller, we may eventually need to move beyond silicon to atomically thin materials, such as the two-dimensional semiconductors we study."

A host of considerations

Size isn't the only consideration; cost is important, too. It's enormously expensive up front to design and build a new chip. "We are as R&D-intensive as drug companies that spend billions of dollars developing one drug," Datta said. "It's a very similar business model."

There are also manufacturing factors to consider. "The often much harder step is producing billions of those transistors reliably, at scale and at a price that makes sense commercially," Persson and Peña said. "Something that works once in a laboratory does not necessarily work in a factory."

Another consideration is energy efficiency. "If I'm adding more and more transistors onto that little piece of silicon, and at the individual transistor level, they don't become more energy efficient … then when I go from, let's say, 10 transistors to 20 transistors, my power budget has to be doubled, all things being equal," Datta said.

For example, the latest Nvidia data center GPUs consume 1.4 kilowatts of electricity, and Nvidia's upcoming release is estimated to consume nearly a kilowatt more than that. Half of that is given up as waste heat, Datta said.

"So it's not just smaller, faster, cheaper, but also more energy efficient," Datta said. "You have to work on all the four vectors."

Is smaller still the goal in the age of AI?

Still, there is a limit to the physical space available for additional transistors, which is why engineers are turning to 3D designs — think taller instead of smaller.

"Instead of only making each transistor smaller, the idea is increasingly to stack transistors on top of one another so that more of them fit within the same chip area," Persson and Peña said. "That way, we can still make our electronics considerably more powerful, even if each transistor shrinks only modestly."

The quest for faster, more efficient chips is only getting more urgent with the explosive growth of AI. But smaller transistors and more powerful chips aren't just important for AI data centers; they also mean smaller, potentially cheaper consumer electronics.

"People certainly notice the consequences of smaller transistors," Persson and Peña said. "They can use less power and allow more computing power to fit on the same chip. That can mean faster electronics, longer battery life and lower costs. Historically, that combination has enabled today's smartphones to outperform room-sized supercomputers from decades ago."

So how small can transistors get? "We will do everything to make it as small as possible as long as we can control the physics," Datta said. "And as long as we can, we will find a way to make the economics work."

The Romans are well known for their aqueducts, which brought water to their cities and settlements.

These aqueducts sometimes ran for very long distances. For instance, the aqueduct of Valens, which brought water to the city of Constantinople (modern-day Istanbul), was 250 miles (400 kilometers) long, according to a statement from Johannes Gutenberg University Mainz.

The aqueducts also lasted a long time; their remains are some of the most prominent Roman architectural finds visible today. All of this demonstrates the Romans' engineering prowess. So how did the Romans build such large and long-lasting aqueducts?

Preparing the route

First, the Romans would select a source of water for the aqueduct, with springs being a popular choice. The aqueducts had to be designed so water would flow toward the city or settlement. The successful building of the aqueducts "was accomplished by an ever slight incline downwards from the spring to its end source," such as Rome, Ann Olga Koloski-Ostrow, a classics professor and chair emerita at Brandeis University in Massachusetts, told Live Science in an email.

Roman surveyors had to design a route carefully. "To choose a route with the necessary gradient, Roman surveyors would have either used the libra or the dioptra," Joseph Lewis, an archaeologist at the University of Cambridge, told Live Science in an email.

The dioptra was shaped like a disc with a sighting bar in the middle. It could be suspended from a stand, and the surveyor could look through the sighting bar at rods in the ground to help determine their elevations. "Surveyors would sight through the sighting bar toward surveying staves to measure differences to elevation," Lewis said.

The exact design of the libra is unclear, but we know that it had a plumb bob that was suspended in the air and the surveyor could compare it to rods in the ground to help determine differences in ground height. "Surveyors would set the libra up along proposed routes for the aqueduct and look forward and backward against calibrated staves," Lewis explained, noting that this allowed them to determine differences in ground height.

Building a long-lasting aqueduct

The Romans used a form of concrete that tended to last much longer than modern-day concrete. Roman concrete used pozzolan, an ashy substance, that was combined with lime and water to form a durable concrete that hardened when exposed to water.

"Roman concrete certainly played an important role, especially in bridges, arcades, and other structural elements," Giovanni De Feo, a professor of industrial engineering at the University of Salerno in Italy, said in an email.

But this wasn't the only factor that allowed Roman aqueducts to last such a long time.

"I believe their remarkable preservation results from a combination of factors," De Feo said. For example, the Romans were careful to select routes that followed stable geological formations.

Regular maintenance that, in many cases, continued into the Middle Ages also helped Roman aqueducts survive the test of time, De Feo noted. This means that they tried to avoid volcanoes and other landscapes that were unstable.

Aqueducts did not always run aboveground; sometimes, they had lengthy underground portions in areas where the landscapes made it difficult to place it aboveground. Some "aqueducts had hardly any stretches above ground outside the city, while others ran on arches for a considerable distance," Gerda de Kleijn, a researcher at Radboud University in the Netherlands, wrote in her book "The Water Supply of Ancient Rome" (J.C. Gieben, 2001).

As the water neared the city, it would sometimes go into pools that helped remove dirt and other visible pollutants. "Water would pool into the basin, sediment would sink to the bottom, then the water would continue from there on its course to the distribution tank," Harry Evans, a professor emeritus of classics at Fordham University, said in an email.

The number of workers required to build an aqueduct depended on its size and timeline for completion. The Hadrianic aqueduct at Athens, which runs for about 12.4 miles (20 km), could have been completed in 15 years by a team of around 500 workers, researchers estimated in a 2022 paper.

Maintaining the aqueduct

After an aqueduct was built, regular maintenance kept it in good shape. Cees Passchier, head of tectonophysics at Johannes Gutenberg University Mainz in Germany, has studied a Roman aqueduct at Divona in France. In a 2023 study, Passchier and colleagues found that carbonate would have built up on the aqueduct over time. Maintenance workers "partly scraped away" the carbonate before replastering the aqueduct, Passchier told Live Science in an email. At some other aqueducts, they wouldn't bother scraping but would "simply plaster over the carbonate without ever cleaning anything," he said.

A sizable number of workers would have been needed to keep aqueducts functional, but it varied based on the aqueduct's size. For "a small place like Divona, a workforce of a dozen people would probably have been enough," Passchier said. "We know that in Rome, there was a staff of a few hundred people to keep the aqueducts working."

Even after the Roman Empire collapsed, many of its aqueducts survived, as some were used and maintained into the Middle Ages and even into modern times. One of the aqueducts, the Aqua Virgo, built in 19 B.C., is still bringing water to Rome today.

From Augustus to Nero, see how much you know about ancient Rome's famous leaders with our Roman emperor quiz!

Every second of every day, Earth is bombarded by particles from outer space. Some of these particles are obstructed by our planet's atmosphere but can spray down other particles, while others can zoom through the atmosphere and easily pass through anything, including us.

You won't feel these particles darting through you, but it raises a question: How many cosmic particles are actually passing through your body at any moment?

The most common particles that stream through us from outer space are ghostly specks known as neutrinos. "An estimated 100 trillion neutrinos pass through each of us every second," Michael Pravica, a professor of physics at the University of Nevada, Las Vegas, told Live Science.

Neutrinos barely interact with regular matter. Nearly all of them pass through us and the rest of the planet without leaving a trace. "The fact that most of these particles never interact with us demonstrates that there is an underlying reality to our existence, much of which we are not even aware of," Pravica said.

Neutrinos originate from nuclear activity. Nearly all of the neutrinos zipping through us come from the nuclear fusion reactions that power the sun, although some neutrinos originate much farther away, even other galaxies, Lu Lu, an assistant professor of physics at the University of Wisconsin-Madison, told Live Science.

Other extraterrestrial particles that continually strike Earth are cosmic rays — electrically charged, highly energetic particles that travel at nearly the speed of light from all directions, according to the University of Chicago. Although the sun's nuclear reactions produce very-low-energy cosmic rays, the vast majority of cosmic rays come from beyond the solar system. These may be created when faraway stars explode as supernovas, matter falls into supermassive black holes, or galaxies collide, per the University of Chicago.

Cosmic rays are mostly either protons, which are the nuclei of hydrogen atoms, or the atomic nuclei of heavier elements (which contain both protons and neutrons), such as helium or iron. Electrons and positrons (the antimatter counterparts of electrons) also make up a tiny fraction of cosmic rays.

When cosmic rays hit Earth, they are generally blocked by molecules of air high in our planet's atmosphere, Lu said. However, these collisions can generate showers of secondary particles that make it to the ground. These include muons, which are negatively charged subatomic particles that are much like electrons except more than 200 times more massive.

At sea level on Earth, about one muon passes through per 0.15 square inch (1 square centimeter) per minute, Lu said. "For a person, the number ranges from tens to hundreds per second," she noted. "Roughly one or two pass through a hand each second."

Because muons are electrically charged, they can interact with matter they encounter. "Their paths can be made visible in a cloud chamber," a box of vapor in which a trail of mist can be seen when a charged particle streaks through the vapor, Lu said.

In contrast, neutrinos are not electrically charged. They also have almost no mass. These two qualities make it very difficult to detect neutrinos, which is why they're often dubbed "ghost particles."

Detecting cosmic particles

Scientists estimate the number of neutrinos that pass through our bodies by recording "the tiny fraction that happen to collide with matter inside or close to a detector," Lu said. "Everything else follows by working backward from those few."

For instance, the IceCube Neutrino Observatory in Antarctica surrounds 0.24 cubic mile (1 cubic kilometer) of ice with thousands of light sensors. "When a neutrino interacts with an atomic nucleus in the ice, it produces fast-moving charged particles, and those particles emit a faint flash of blue Cherenkov light, which the sensors record," Lu said. "From the pattern and brightness of that light, we can reconstruct the neutrino's energy and direction."

To estimate how many neutrinos pass through Earth based on these recorded interactions, "we account for the size of the detector, how long it operated, its detection efficiency, and the probability that a neutrino of a given energy interacts at all," Lu said. "It is a bit like estimating the number of fish in a river from the few caught in a very coarse net. If you know the size of the net, how long it was in the water, and how easily fish slip through it, you can work backward to the whole river."

When it comes to the neutrinos that IceCube studies, although trillions pass through each of our bodies every second, nearly none of them interact with our atoms. "You would expect no more than roughly one interaction in your body, perhaps a few, over an entire lifetime," Lu said.

These rare interactions would have virtually no effect on us. Given how little energy such an interaction would deposit, such neutrinos "pose no health risk at all," Lu said.

In contrast, the showers of particles resulting from cosmic ray impacts can potentially affect human biology. "At ground level, muons dominate the penetrating charged particles reaching us from cosmic-ray showers," Lu said. "At higher altitudes, and especially during air travel, other secondary particles, particularly neutrons, contribute a significant share of the biologically relevant dose."

Still, any effects from these cosmic-ray showers are generally small. "For an average person, cosmic radiation contributes roughly 0.4 millisieverts per year, about the same as a few chest X-rays spread across the year, out of a total natural background of about 2.4 millisieverts from all sources," Lu said. "The dose varies with altitude, latitude and shielding. At typical ground level, muons and other cosmic-ray particles are not considered a significant health hazard. Life on Earth has been bathed in them from the beginning."

So, every second, there are about 100 trillion neutrinos and tens to hundreds of muons passing through your body per second, and maybe a tiny scattering of neutrons and other cosmic particles or debris from cosmic particles impacting the atmosphere.

These cosmic particles may lead to a variety of discoveries. For instance, scientists have used muon detectors to identify hidden passageways in pyramids. And "high-energy neutrinos allow us to study some of the most violent and otherwise hidden environments in the universe, such as the regions surrounding black holes, exploding stars and other powerful cosmic accelerators," Lu said.

"Because neutrinos interact so weakly, they can escape from dense regions that light may not be able to penetrate," she explained. "They therefore carry information directly from the places where some of nature's most energetic particles are produced."

All in all, "the particles streaming through us are a constant reminder that we are not separate from the universe," Lu said. "We are part of a cosmic story stretching back 13.8 billion years."

See how much you know about black holes with our black hole quiz!

There's a commonly cited idea that water in a drain swirls in opposite directions in the Northern and Southern hemispheres. But is there any truth to it? When you pull the plug from a bathtub, does the water circle one way in New York and the other way in Sydney?

Although the idea is based on a real scientific principle, you probably won't see it play out in a small drain in your house, experts told Live Science.

The Coriolis effect

The idea that water spins in opposite directions in the Northern and Southern hemispheres stems from a phenomenon known as the Coriolis effect. When something, including air or water, moves across Earth as it rotates on its axis, that substance appears to curve because the ground beneath it is rotating at different speeds in different places. Points close to the equator move faster than points near the poles because they have a bigger circle to travel in the same 24 hours.

Over the immense distances covered by weather systems and ocean currents, that small deflection can accumulate and become important. The key words here are "immense distances." The Coriolis effect is weak, so it needs enough time and space to produce a noticeable change. Storms and weather patterns can persist for days and stretch across hundreds of miles. But water in a sink, bathtub or toilet has only seconds to move before disappearing down the drain.

"You always need to think about the right timescales and length scales," Susanne Horn, a professor of numerical and mathematical fluid dynamics at Coventry University in the U.K., told Live Science.

At your bathroom's scale, seemingly trivial details take control. The water may already be rotating slightly because of how it entered the basin, how somebody moved in the bath or how the plug was removed. The sink and drain are unlikely to be perfectly symmetrical, which can also play a part, and the design of a toilet deliberately guides its water. All of these factors are more influential on your basin than the Coriolis effect is. (In other words, that episode of "The Simpsons", which features Bart investigating drainage direction in Australia versus the U.S., got it wrong.)

Consequently, the same sink can drain clockwise on one occasion and counterclockwise on another, depending on the variation of these variables on any given day; the direction depends principally on how the water was introduced and the geometry of the container and drain, according to the Library of Congress.

So, although the Coriolis effect behind the claim is real, the water in a bathroom drain is normally overwhelmed by much stronger influences.

How to see the Coriolis effect in action

To make Earth's rotation ‪—‬ and, thus, the Coriolis effect ‪—‬ matter in a drainage experiment, Horn said, "there are essentially two tricks: let it drain very slowly, or make it much bigger."

Fluid dynamicists describe this using the Rossby number, which compares a flow's inertia with the influence of the planet's rotation. A small Rossby number indicates the Coriolis effect is important; a large one means the flow is moving too quickly or across too short of a distance, for Earth's rotation to have much influence. A typical household drain falls firmly into the latter category. In Horn's words, it "doesn't feel rotation."

Still, there are ways to see the Coriolis effect in action. In a meticulously designed experiment researchers suppressed those larger disturbances to give the Coriolis effect enough time to emerge.

In the 1962 experiment, described in the journal Nature, an MIT researcher used a large, carefully prepared tank that removed residual movements in the water before it drained. The water eventually developed the counterclockwise rotation predicted for the Northern Hemisphere.

Three years later, in another study published in Nature, researchers at the University of Sydney published a Southern Hemisphere counterpart. After taking similarly stringent precautions against outside disturbances, they observed the predicted clockwise rotation. The two experiments demonstrated that Earth's rotation could select the direction of a draining vortex — but only under extremely well-controlled conditions. In other words, the Coriolis effect appears only once you've removed practically every other variable.

That is very different from saying every sink or toilet flows in the same direction according to which hemisphere they're in. In normal plumbing, the drain's shape, the water's starting motion and the way it is released are all far more influential.

"Just ask yourself whether what you're looking at happens slower than Earth's rotation and for long enough, only then does the Coriolis force really start to matter," Horn said.

Every person you know has a name, and you've likely given one to your pets, too. But are names a uniquely human feature, or do other animals also call each other by name?

Research suggests that some mammals and birds use vocal labels for other individuals, but not all scientists agree on how to interpret the evidence.

In the 1960s, scientists discovered that young bottlenose dolphins (Tursiops) create individually distinctive whistles, called signature whistles. Researchers in the years since have conducted experiments in which they played the whistles through an underwater speaker and observed how the dolphins responded. Dolphins copy the signature whistles of other dolphins to initiate contact, similarly to how we use names, and can recognize other individual dolphins based on the frequency and "shape" of the whistle, regardless of the voice of the dolphin whistling.

Dolphins do seem to assign these whistles to other individuals and to match the whistles to others in a more symbolic way, Kelly Jaakkola, a cognitive psychologist at the Dolphin Research Center in Florida, told Live Science. Research shows that dolphins whistle to identify each other within complex social systems and while cooperating on tasks.

Through playback experiments, researchers also found that wild African elephants (Loxodonta) address one another with specific vocalizations ‪—‬ low rumbling sounds that are unique to each individual. The team published their findings in 2024 in the journal Nature Ecology & Evolution.

"They sound a little bit like a giant cat purring," Michael Pardo, a behavioral ecologist at Colorado State University and first author of the study, told Live Science. Elephants tended to approach speakers more quickly, and vocalize more in response, when their "name" was played.

Playback experiments have also suggested that spectacled parrotlets (Forpus conspicillatus), small parrots native to parts of South and Central America, use vocal labels for other parrotlets.

Studies of birds in captivity can shed light on their capacity to learn vocal labels. In a survey of parrot owners, about half of the 800 birds used vocal labels for individual people and animals that they had learned from the people around them.

"Parrots certainly pick up a lot of things from humans — they don't necessarily know what it means in a human linguistic context, but they know when to say it," Lauryn Benedict, a biologist at the University of Northern Colorado and first author of the study that surveyed parrot owners, told Live Science.

One survey participant said their parrot corrects people when called by the wrong name; others said their parrots correctly used the names of all humans and dogs in the household. However, these behaviors look different from those in wild parrots, and further research is needed to dig into whether spectacled parrotlets understand the concept of names and actually give names to their fellow birds.

How do you define a name?

Claiming an animal noise is a name, as humans define it –– a specific, symbolic vocal label attached to an individual –– is complicated.

One study in marmosets suggests that they use vocal labels. Using playback experiments, the researchers discovered that the marmosets directed certain vocalizations at specific individuals and that they learned these vocalizations from family members. However, in a published response to that study, Jaakkola argued that the calls may not be name-like, since they were correctly identified only 16% to 61% of the time. Jaakkola proposed that the marmosets may have been imitating each other's vocalizations.

However, she noted that we can't generalize any one interpretation to all species.

"The elephant case is very different from the marmoset case and the spectacled parrotlet case," Jaakkola said. "It can be really hard to have a cohesive discussion [across species] ... to all talk about the same thing."

In a July 2026 paper, Jaakkola and colleagues proposed three minimum criteria to apply across species: individual reference, meaning the call is designated for one particular individual; symbolic representation, meaning the name stands for a specific individual in the user's mind; and shared meaning, where at least two individuals associate the name with that individual.

When applying these criteria to the current evidence, Jaakkola said bottlenose dolphins show the strongest evidence for the use of animal names. Elephants, parrots and marmosets, on the other hand, require more research about symbolic representation to prove naming behavior.

"It is a big claim when you call something a name," Jaakkola said. "Our goal here was to create a unified framework … to start looking at this question more systematically."

Why are names useful?

Species that may use name-like calls have a couple traits in common: They can learn new sounds, and they're extremely social. Names might be useful in animal groups in which the community breaks into smaller subgroups and individuals are often out of their companions' sight, Pardo said.

The behavior seems present in very different lineages, so it may have evolved separately multiple times, which can tell scientists about the evolution of language among complex social pressures, Pardo said.

By studying whether animals use names, scientists can learn about the cognitive abilities necessary for abstract mental representation of other individuals. "For humans, names are such a part of our individuality and our sense of self, that if animals have names and respond to those names, that suggests they could have a sense of self … and of other individuals and their individuality," Benedict said.

Jaakkola clarified that although it's difficult to demonstrate naming behavior in nature, she's not ruling out that nonhuman animals have name-like calls. Rather, the evidence is not yet sufficient to say they do, she said.

Pardo agrees that more evidence would confirm these behaviors in more species, but he suspects a lot of animals may be calling each other by name.

A lot of other animals, such as baboons, giraffes, and crows, have communities that break into smaller groups. While there isn't evidence for names in species like these, and this social structure would need to be paired with vocal learning abilities, "I think it might be more common than we currently realize," Pardo said.

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We all know that a dropped or shaken soda can comes with the risk of a fizzy eruption. People use different techniques to prevent these bubbly blasts, including tapping the top of the can. This advice has been passed around for years, but does it actually work?

The answer lies in the physics of what's happening inside the can.

Building the bubbles

To make a carbonated drink, carbon dioxide is injected into the liquid — such as soda, beer or Champagne — at a high pressure and cold temperature.

"[Bubbles] are formed as a result of what is called supersaturation of carbon dioxide," Roberto Zenit, a professor of engineering at Brown University who studies fluid mechanics, told Live Science.

Supersaturation is an unstable state where a liquid or gas can hold more dissolved materials than normal conditions allow. Thanks to its instability, there are some actions, like shaking a can, that can encourage more bubble production.

"The mechanical action (tapping or shaking) generates violent pressure waves inside the liquid; these waves rebound at the walls, creating low-pressure zones where tiny bubbles form," Javier Rodríguez Rodríguez, an associate professor of fluid mechanics at Carlos III University of Madrid, told Live Science in an email. "If given enough time (minutes to tens of minutes, depending on the drink and container), these bubbles dissolve again and the can 'forgets' that it has been perturbed."

However, if the soda can is open while the bubbles are suspended in the liquid, the sudden pressure drop from opening the can encourages more bubble production, resulting in more spillover, Rodríguez Rodríguez said.

In some rare cases, like spraying Champagne after a NASCAR victory, creating a geyser of foam is exactly the goal, which the drivers accomplish by vigorously shaking the bottles beforehand.

Does tapping actually work?

Unlike NASCAR drivers, most people are looking to save their drink‬, so they may try tapping the top of the can.

The idea behind this strategy has some merit. According to Zenit, carbon dioxide bubbles can expand out of tiny imperfections called nucleation sites, which can include dust particles or rough spots on the can.

"The reason why you tap is basically to remove possible bubbles or dust particles within the soda can," Zenit said. If you remove those, you can reduce the rate of bubble formation, he added.

In theory, a few gentle taps could knock loose some bubbles clinging to the walls of the can, allowing them to float into the pocket of air above the soft drink before it's opened. With fewer nucleation sites left in the liquid, there would be fewer places for bubbles to rapidly grow once the can was opened and the pressure dropped.

But that explanation comes with an important caveat: While gentle tapping might dislodge a handful of bubbles, tapping too hard, or tapping repeatedly, could create more bubbles, Rodríguez Rodríguez said. The same principle applies to carbonated drinks in glass or plastic bottles, as tapping down on a beer bottle can encourage foam to fizz over.

Those competing effects make it difficult to determine whether tapping actually helps, and some experiments suggest that overall, it probably doesn't.

In one preprint study from 2019, researchers tested 1,031 cans of beer by dividing them into four groups: shaken and tapped, shaken and untapped, unshaken and tapped, and unshaken and untapped. The tapped cans were flicked three times on the side with a finger before being opened. The researchers found no statistically significant difference in the amount of beer lost between the tapped and untapped cans, regardless of whether they had been shaken. They concluded that tapping "does not prevent beer loss" and that the practice isn't supported by their observations.

That doesn't necessarily settle the question for soda, but it hints that tapping is not an effective method.

"Beer and soda differ in composition and foaming behavior, but the study provides relevant experimental evidence that tapping is not a demonstrated solution," Kit Yam, a food scientist at Rutgers University, told Live Science in an email.

How do you prevent a soda can from fizzing?

If tapping isn't the answer, time probably is. Letting a shaken can sit upright for several minutes gives the bubbles time to dissolve or rise into the air pocket at the top of the can. Keeping the drink cold can also help, because carbon dioxide stays dissolved more easily at lower temperatures.

"Put the can you accidentally dropped or hit in the fridge for half an hour and open another one in the meantime!" Rodríguez Rodríguez said.

If you don't have another can or you don't want to wait, Rodríguez Rodríguez suggested another option: opening the can slowly.

"In this way, the gas can escape, but I only leave a tiny slit for foam to fizz out," he explained. "Then, when you hear the gas is no longer escaping, you can safely open the can completely."

You can use the same process to reduce the amount of foam that spills over when you pour a carbonated drink into a glass.

"If you pour it slowly," Zenit said, "then you reduce this agitation."

Scientists have discovered the basic ingredients for life in asteroids and meteorites, on Mars, and around young stars. All of those findings have come in the past few years, and upcoming missions aim to search for these building blocks of life.

But how many building blocks of life have we found in space so far?

First, it's important to define what we mean by the building blocks of life. Scientists are looking for molecules that look like those found in life on Earth, including proteins; RNA; and lipids, the fatty molecules that make up cell membranes.

We’ve found many on sample-return missions. The asteroid Bennu, brought back to Earth in 2023, contained all five nucleobases of DNA and RNA, along with 14 of the 20 amino acids used by living organisms. Its sibling asteroid Ryugu also yielded all five nucleobases, including uracil, which is found in RNA.

Other techniques, like radio astronomy, look for these building blocks in distant regions of space.

Astronomers categorize the molecules they find by complexity, starting with a broad group called "complex organic molecules" (COM). These are carbon-containing molecules with six or more atoms. Of the 350 molecules of any kind discovered in space so far, "about 180 of them qualify as COMs," Sergio Ioppolo, an astrochemist and an associate professor in the Department of Physics and Astronomy at Aarhus University in Denmark, told Live Science. "That cutoff is an operational convention rather than a fundamental definition of complexity."

Not all COMs are building blocks of life, but some of them are. A subgroup within COMs is a "prebiotic" molecule that scientists think may have played some role in the origin of life. A prebiotic molecule "has no structural rule," Ioppolo said — different scientists define it slightly differently.

Ioppolo defined a prebiotic candidate as a molecule whose detection scientists are confident in, that could plausibly form and survive without life already existing, and that has a documented role as a building block, precursor or intermediate in some proposed pathway to life.

Using that definition, Ioppolo estimates "roughly 30 detected prebiotic candidates, although the exact number depends on where the line is drawn."

Some of the richest hunting grounds for these molecules are in molecular clouds ‪—‬ interstellar regions of gas and dust that are dense enough for molecules to form. Using radio astronomy, Izaskun Jiménez-Serra, an astrochemist and staff researcher at the Center for Astrobiology in Spain, and her team have detected more than a dozen prebiotic molecules in molecular clouds at the center of the Milky Way, including the four-carbon sugar erythrulose — the first true sugar found in space.

All molecules great and small

However, there's a limit to the size and complexity of molecules that astronomical surveys can detect. To identify each molecule, scientists analyze the light from molecular clouds, which can reveal which parts of the electromagnetic spectrum have been absorbed. Those "spectral lines" act as a fingerprint for each molecule.

"And then we say, 'OK, this line should be that molecule,'" Ioppolo said. "When you're talking about simple molecules, that's easy. When you're talking about larger species [molecules], they have absorption bands that overlap with each other. So it's basically impossible to distinguish between one species and another because they will all have the same spectrum."

That limit means that even with the large numbers of prebiotic molecules we've already found in space, there are probably a lot more out there. But if the building blocks of life keep turning up everywhere we look, does that make it less impressive when we find them in places like Mars?

Ioppolo said no. "We still don't have answers to questions such as, is the material that we inherited for those early stages preserved through the formation of stars and planets? Throughout that process, you have a lot of things happening," he said. For instance, high temperatures, cosmic rays, X-rays and ultraviolet light all have the potential to destroy complex molecules. "So when you hear that we found some amino acids on the surface of Mars, I will still be impressed," Ioppolo said.

Which molecules are we still looking for?

There's also a bigger question: Does the abundance of these ingredients in space mean that life itself is common in the universe?

"These are very small molecules," Jiménez-Serra said. "I don't want people to say that I've said life was detected."

However, she's excited by the possibility that molecules of even greater complexity can form in the punishing environment of space. "Under the very extreme conditions of interstellar space with very low temperatures, close to absolute zero, and very low pressures, basically vacuum pressures, you can find these key ingredients for life," she said.

"This encourages us to continue looking for more of these molecules — for larger sugars, for instance, like ribose, [which] is part of RNA," she said. "If we can find ribose, that will mean that all of these key ingredients for life can form even before the stars and planets are born."

While radio astronomers survey the skies for these basic ingredients, upcoming sample-return missions have the potential to uncover larger, more complex molecules — maybe even RNA itself. Nearly 10 missions have launched or are planning to launch in order to land on rocky bodies in space and collect samples. Japan’s Hayabusa2, which launched in 2014 and has already collected samples from the asteroid Ryugu, is now on its way to two more asteroids. And China’s Tianwen-2 mission launched in 2025 to collect samples from the near-Earth asteroid Kamoʻoalewa.

"The fact that [prebiotic molecules] are everywhere in star-forming regions means that there is a very high chance that if the conditions are right, you could potentially start the process of formation of life in other places," Ioppolo said. "We do know that all the ingredients are out there, but so far we only know that we are out here. We don't know if it worked out for someone else as well."

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If you've ever been jolted awake by your cat letting out a long, mournful cry in the middle of the night, your sleep-deprived brain may wonder why, exactly, your cute little pet yowls all the time.

The answer isn't as simple as "they want food" or "they're being dramatic." The sound people commonly call a yowl can actually mean very different things depending on the situation, and even cat behavior experts don't always agree on exactly what qualifies as a yowl.

In veterinary medicine, the term often refers to loud, harsh vocalizations associated with aggression or mating between cats. But many pet owners use "yowling" to describe almost any loud cry their cat makes at home, she said.

That distinction matters, because context is often the biggest clue to what a cat is trying to communicate.

"If you're talking about a yowl in the strict sense, it's usually linked to a strong negative emotional state," said Charlotte de Mouzon, a cat behavior researcher and ethologist at Paris Nanterre University in France. "It represents an emotion that is intense and negative."

In other words, a true yowl is less like casual conversation and more like an emotional alarm bell.

An ancient warning system

That alarm bell may be older than the house cat (Felis catus) itself and likely predates domestication entirely, said Jay Schwartz, a professor of psychological sciences and head of the Evolution of Emotional Communication Lab at Western Oregon University.

The same vocalization shows up in the African wildcat (Felis lybica), the domestic cat's closest living relative that has been around far longer than our living room tigers — meaning cats were yowling at rivals long before they ever curled up on a human's couch, he told Live Science in an email.

Territorial vocalization of this kind is common across the animal kingdom, from a chickadee's warning whistle to a howler monkey's booming roar, Schwartz said. Establishing and defending territory is tied to food, safety and mating opportunities, so animals evolved loud, unmistakable signals to defend it without resorting to physical fights every time.

A yowling cat squaring off with a neighborhood rival, in other words, is running a very old evolutionary script.

Fear, conflict and distress

Beyond territory, cats may yowl when they're frightened, cornered or trying to make another animal, or person, back off.

"If someone approaches a cat that's yowling, I would think that cat is saying, 'Please don't come any closer,'" said Katherine Anderson, a board-certified veterinary behaviorist at the Cornell Duffield Institute for Animal Behavior at Cornell University.

The same vocalization can happen during stressful situations, such as a trip to the veterinarian, when a cat is restrained, or during conflicts with other cats. Female cats in heat may also produce loud yowls, as can males competing for mates, Anderson told Live Science.

While humans often interpret these cries as a cat "screaming," de Mouzon said they're best understood as expressions of intense emotional distress. Sometimes the vocalization is directed at another individual, while other times it's simply an outward expression of the cat's emotional state — not necessarily a conscious plea for help.

Cats have learned that humans listen

Not all loud vocalizations are classic yowls, however. Cats have an impressive repertoire of sounds, and many of the noises owners hear every day are part of a unique form of communication that developed alongside humans.

Adult cats rarely meow to one another in natural settings. Instead, meowing is mostly reserved for kittens communicating with their mothers. Domestic cats appear to have retained this juvenile behavior into adulthood and redirected it toward people, de Mouzon said.

Schwartz noted that African wildcats also meow, but in a harsher, more yowl-like way than domestic cats do, and typically during conflict with other cats. One hypothesis for why the domestic cat's meow softened over generations is that it happened to share acoustic similarities with a human infant's cry, making nearby humans instinctively more responsive to it.

"They learn that meowing is useful for communicating with humans because it gets our attention," de Mouzon said.

Anderson agrees that cats have become remarkably skilled at shaping human behavior. "I think they learn our behavior faster than we learn theirs," she said. "They can train us much easier than we can train them."

That learning goes both ways. If a cat cries at 5 a.m. and its owner immediately fills the food bowl, the cat quickly learns that vocalizing produces results. Over time, the behavior may become more frequent — not because the cat is trying to be annoying, but because it has learned that the strategy works.

Why "yowl" is such a slippery word

Part of the confusion around yowling may come down to how humans classify sounds in the first place. In a 2025 study published in the journal Animal Behaviour, Schwartz's lab had participants listen to audio clips of cat vocalizations pulled from online sources and sort them into categories like meows, chirps and yowls.

Some categories were easy to define — clips people labeled "chirps," for instance, were consistently short and high-pitched. Yowls were a different story. The clips people called yowls varied wildly in pitch and length, and listeners agreed with each other less on what counted as a yowl than on almost any other vocalization type.

Schwartz suspects that's because people aren't classifying yowls by acoustic features at all — they're classifying them by the emotion they perceive. If a sound comes across as distressed or upset, listeners are likely to call it a yowl, regardless of its pitch or duration. It's similar to how a person might describe someone as having "yelled" versus "shouted" based on the emotional tone they picked up on.

That fuzziness may explain why familiarity with an individual cat matters so much. A 2015 study found that owners were fairly accurate at identifying the context behind their own cat's vocalizations but far less accurate when listening to a cat they didn't know.

When yowling could signal a health problem

Although vocalization is often perfectly normal, a sudden increase in loud crying, especially in an older cat, can sometimes point to an underlying medical condition.

Two of the most common concerns veterinarians consider are hyperthyroidism, which causes excessive thyroid hormone production, and cognitive dysfunction syndrome, a form of age-related cognitive decline similar to dementia in humans. Both conditions commonly cause nighttime vocalization.

Age is one of the first things Anderson considers when owners report excessive yowling. Cats may also vocalize because they're anxious, experiencing separation-related distress or in pain.

Rather than focusing on the sound alone, veterinarians recommend looking at the whole picture, such as when the vocalization is happening, and changes to the cat's routine.

They're not trying to spite you

One of the biggest misconceptions Anderson encounters is the idea that cats vocalize or engage in other unwanted behaviors to punish their owners.

"I don't think I've ever had a case where I'm like, 'Yep, that was spite,'" she said.

Instead, behaviors that frustrate owners usually have simpler explanations. A cat that cries constantly may have learned that vocalizing earns food or attention. Another may be fearful, anxious or experiencing an illness that isn't immediately obvious.

Cats may not communicate the way humans do, but they're almost always communicating something. The challenge isn't getting them to speak, it's learning how to interpret what they're already saying.

How much of a cat fan are you? Find out by taking our cat quiz!

Although it might be difficult to believe in our hyper-connected lives, there are thousands of people across the planet who remain almost completely disconnected from the rest of the world.

It's estimated that there are at least 196 "uncontacted" Indigenous groups worldwide, living mainly in 10 countries in South America, Asia and the Pacific, including Brazil, Indonesia and India. They live in diverse environments, from islands, to deserts, and the forest-dense borderlands of the Amazon basin. These groups have had varying levels of contact with the outside world; while some have only sporadic encounters with neighboring Indigenous peoples, others more regularly encounter outsiders, sometimes against their will due to the presence of oil, logging, agriculture and mining industries that increasingly encroach on their lands.

But who among them are the most isolated people on Earth?

While this question is difficult to answer, there are some candidates for the "least contacted" groups, experts told Live Science.

There aren't strict definitions of what makes a group "uncontacted." The United Nations even notes that the description of such groups can vary from "free, uncontacted, hidden or invisible peoples, peoples in voluntary isolation, etc." Hal Russell, Asia and Pacific research and advocacy officer with the nongovernmental organization, Survival International, said, "We define 'uncontacted peoples' as Indigenous peoples who generally avoid contact with outsiders and have no permanent relationship with them, even while being aware of the outside world."

"Certainly there is a spectrum between permanent contact, initial contact, sporadic contact and none at all," he told Live Science.

Generally what unites these groups is that they have actively avoided or tried to limit contact with other cultures, even with other uncontacted peoples, to maintain their unique way of life.

"They do not want to be disturbed"

One such candidate is a small group of Indigenous islanders known as the Sentinelese, who live on North Sentinel Island within India's Andaman and Nicobar Islands in the Bay of Bengal. This group is so little known that anthropologists aren't even sure what they call themselves; their name is based on the roughly 23-square-mile (60 square kilometers) island where they live.

According to Survival International, it's believed that the Sentinelese have lived in the Andaman Islands for up to 55,000 years and are direct descendants of the first humans to migrate out of Africa. Today, the Sentinelese follow a hunter-gatherer lifestyle and fish along the coastline, using sleek, hand-carved canoes. Survival International estimates that the group includes between 50 and 150 people, although lack of access to the group makes them near impossible to count, and the government has not attempted to include them in the Indian census. Their official numbers remain unknown.

Aside from the island's remoteness, contact is even more difficult because the islanders reject it. The community has been known to shoot at approaching boats and hovering helicopters.

"When they shoot bows and arrows, they are communicating," Simron Singh, a professor at the University of Waterloo in Ontario whose expertise is in island environments and the peoples of the Andaman and Nicobar islands, told Live Science. "It says they do not want to be disturbed."

Only a handful of outsiders have ever set foot on North Sentinel Island, and Indian law now prohibits travel within a 3-mile-radius (5 km) of the island for the protection of both parties. Islanders are vulnerable to diseases brought by outsiders, and visitors are at risk of attack; the Sentinelese have shot and killed at least three people, including a missionary named John Allen Chau, who traveled to the island illegally in 2018.

"They erase their footprints"

Survival International estimates that 95% of "uncontacted" and "isolated" peoples live in the Amazon Basin (though the term "voluntarily isolated peoples" is more commonly used to refer to such groups there, according to Beatriz Huertas, an anthropologist, independent researcher and special adviser on isolated peoples policy with Rainforest Foundation Norway).

Huertas conducts research on these groups and to her one stands out in particular: the Kakataibo, a forest peoples in the Aguaytia basin, part of the Amazon in central Peru. The Kakataibo are very rarely encountered, even by other Indigenous peoples. In fact they "are so determined not to be found that they erase their footprints with Earth," Huertas said, "so there is not much information about them."

"We know that this isolation is a protective strategy they have adopted" precisely because they have had contact with the outside and it has not been a positive experience, Huertas explained.

Survival International highlights other highly-isolated groups in the region, including a nomadic Indigenous group called the Kawahiva. This group has evaded contact so successfully that they are known only through chance encounters and one piece of video evidence, and were only recognized as a distinct group in 1999. It's not uncommon for there to be "little recognition or documentation of an uncontacted group, in fact there are likely many uncontacted groups with no documentation at all," Russell said.

He noted that even in Brazil which has some of the most rigorous documentation policies, one Indigenous group was only documented in 2021, having eluded authorities up until that point. Very little is known about them, including their name.

Another prominent group that is extremely isolated is the Mashco Piro of the remote forests of Peru, who are thought to constitute the largest group of uncontacted peoples in the world, numbering about 750. Some in this group are the descended relatives of people who were enslaved and forced to work on Amazonian rubber farms in the 1800s, but escaped deeper into forests where they have since remained, shunning contact.

Misconceptions about isolated peoples

This history of contact for even the most remote people on Earth shows that the term "uncontacted people" is misleading, Huertas said. In fact, the phrase has fed into unhelpful misconceptions that these groups are "stuck in the Stone Age, as if they were living in a bubble," she said. "In the social sciences, we know that no people remain completely isolated from what is happening in their surroundings and, therefore, no people have remained stuck at any particular stage in history."

Another frequent misunderstanding is that these groups would want to connect if they knew more about the benefits of the outside world, Singh said. He has worked with the Nicobarese peoples of the Nicobar Islands, who haven't completely rejected contact like their Sentinelese neighbors, and has discovered that in their case this is not true. In fact, the Nicobarese are aware of wars and conflict beyond the island, and this has framed their perception that the outside world is "dangerous and uncivilized," Singh said.

In many cases historical experiences can explain why these groups would want to avoid the world beyond. By the mid-1900s, the Great Andamanese of nearby islands had experienced a population decline of up to 99% after bouts of conflict with British settlers and the influx of disease. The Sentinelese themselves had a historical encounter with British colonizers in the 1800s during which time some were kidnapped, infected with disease, and died, which may have shaped their current wariness of the outside world.

Some researchers have argued that these groups should be contacted to better protect them from current and future harms. But groups like Survival International counter that a long history of forced contact shows that this ultimately leads to more harm than good for these peoples, their safety and their survival.

Indigenous peoples have a human right to self-determination as affirmed by the United Nations, and researchers like Huertas and Singh said these groups clearly express that right by seeking contact when they want to associate with others, and retreating when they want to be left alone.

"It's not that they don't have the option [to seek contact] ... It's not rocket science to get on a boat and sail somewhere," Singh said. "It's a choice not to connect."

Yams and sweet potatoes are often confused, and the names are sometimes used interchangeably in the U.S. However, they're two different crops. So what sets them apart?

The differences lie mainly in these vegetables' classifications, plant growth, and nutritional profiles, experts told Live Science.

Despite both growing underground and appearing somewhat similar, "Botanically, sweet potatoes and yams are not closely related, as they belong to different plant families," said Phillip Wadl, a research geneticist at the U.S. Vegetable Laboratory in Charleston, South Carolina. Sweet potatoes (Ipomoea batatas) are in the morning glory family (Convolvulaceae). Yams (Dioscorea), by contrast, belong to the Dioscoreaceae family, and there are 11 yam species cultivated for food and income, Wadl told Live Science in an email.

Yams and sweet potatoes also belong to different plant groups, or "cotyledon types," said Jeffrey Cole Denis Gregorie, a sweet potato specialist at the Sweet Potato Research Station in Chase, Louisiana. Yams are monocots, plants whose embryo has only one cotyledon, or seed leaf, like grasses and lilies. Sweet potatoes are dicots, plants with two embryonic seed leaves, which include many fruits and vegetables, such as grapes, squash, strawberries and tomatoes.

In addition, yams and sweet potatoes store food in different parts of the plant. "Sweet potatoes form storage roots which are enlarged parts of the root system," Wadl explained, "whereas yams form tubers, which are enlarged underground stems for nutrient storage."

A single sweet potato plant typically produces about four to 10 sweet potatoes, while a yam plant yields about one to five yams. Sweet potatoes generally take about three to five months to mature, compared with about six months to a full year for yams, according to Craig Yencho, a professor of horticultural science and a sweet potato and potato researcher with North Carolina State University.

How do you tell sweet potatoes and yams apart?

Visually, sweet potatoes and yams are easy to tell apart when you know what to look for. Sweet potatoes tend to be shorter and blockier, with tapered ends. Yams are usually long and cylindrical, sometimes with "toes," or branching shapes.

Sweet potatoes come in various flesh colors, including orange ‪—‬ the most common color in the U.S. ‪—‬ as well as yellow, purple and white. Their skin is typically thin and smooth, whereas "yams often have a rough, bark-like skin and their flesh can be white, purple or reddish," Wadl said. One well-known purple yam, known as ube (Dioscorea alata), is a popular ingredient in treats such as ice cream, cookies and pound cake.

Sweet potatoes and yams also have noticeable differences in their culinary and nutritional profiles, Wadl said. "Sweet potatoes are generally sweeter and have a moist, creamy texture when cooked, although dry types are available," he explained. "Yams, on the other hand, tend to be starchier, drier, and have a more earthy, sometimes less sweet, flavor compared to sweet potatoes. Their texture can be more like a regular potato."

Nutritionally, the two crops are quite different, too. Sweet potatoes, particularly the orange-fleshed varieties, are an excellent source of beta-carotene (a precursor to vitamin A), vitamin C, dietary fiber, potassium and some B vitamins. "While nutritious, [yams] typically contain less beta-carotene than orange sweet potatoes," Wadl noted. Instead, yams are a good source of potassium, vitamin C, manganese and dietary fiber.

Sweet potatoes are generally considered safe to eat both raw and cooked, Wadl said. By contrast, many wild varieties of true yams, especially those not cultivated for commercial consumption, contain compounds like dioscorine that could make them toxic to humans if eaten raw. "These must be thoroughly cooked to remove the toxins and make them safe for consumption," Wadl said.

Thanksgiving confusion around yams and sweet potatoes

If yams and sweet potatoes are so different, why do so many Americans call sweet potatoes "yams"?

Part of the answer lies in history. The yam was a key provision on voyages of enslaved people from West Africa to the Americas, "used to feed captives a familiar food," said Judith Carney, a professor of geography at UCLA. "In mainland plantation societies of North America where the African yam could not be grown, the sweet potato served as a substitute dietary staple." Because the two crops shared some characteristics, "the word 'yam' came to refer vernacularly to both tubers in mainland America, as we see in the U.S. today," Carney told Live Science in an email.

The U.S. sweet potato industry also leaned into the confusion, adopting the term "yam" for marketing reasons, Gregorie told Live Science in an email. "In the United States, many orange-fleshed, moist varieties of sweet potatoes are often mislabeled as "yams" to distinguish them from the drier, white-fleshed sweet potato varieties," Wadl explained.

As a result, the "yams" at most U.S. supermarkets are probably sweet potatoes. "Garnet yams" and "jewel yams" aren't actually yams, despite those labels; instead, they're softer, sweeter varieties of sweet potatoes, according to the University of California, Davis.

Although genuine yams are sold in many international markets in the U.S, "if you're buying a "yam" in a typical U.S. supermarket, you're almost certainly purchasing a sweet potato," Wadl said.

Do you know where pumpkins and blueberries come from? Find out with our fruits and vegetables quiz!

The ancient Celts thrived in Europe, earning a reputation as fierce warriors. They sacked Rome around 390 B.C. and, decades later, were received by Alexander the Great while he was conquering his empire.

It's no surprise, then, that archaeologists find many weapons in Celtic graves. However, over the decades, some of those weapons have been found in burials of women, hinting that they may have used these weapons. What's more, recent genetic studies suggest that elite Celts inherited power through maternal lines.

The Celts lived throughout Europe, from what is now Spain to Turkey. They shared a similar language and culture, and clues of female power have been found throughout their old lands and hinted at in ancient texts. This raises an interesting question: Did the Celts have women who fought in battle?

Opinions vary, but some experts Live Science interviewed thought it was possible that Celtic women could have fought in battle, although perhaps not regularly. One problem is that there is an ongoing debate among scholars as to whether the inhabitants of the British Isles around 2,000 years ago were, in fact, Celtic.

Genetic hints

Although it's not directly related to answering the question about Celtic women warriors, a few recent genetic studies have found that elite Celtic women held powerful roles. A 2024 study in the journal Nature Human Behaviour that looked at ancient DNA in several Celtic burials from Germany found a matrilineal inheritance of power, and a 2025 study of burials from Iron Age England revealed that men married into local, powerful female lineages.

This supports textual evidence that Celtic women had more power than many of their female counterparts across Europe. Women "of the Celts did generally enjoy more freedom and opportunity than their counterparts in the Greek and Roman world," Philip Freeman, a humanities professor at Pepperdine University, wrote in his book "War, Women and Druids: Eyewitness Reports and Early Accounts of the Celts" (University of Texas Press, 2002). "It is hard to imagine a Greek wife joining her husband on the battlefield or even leading an army, as did the British queen Boudicca."

"Flashing-eyed" women

Another piece of possible evidence is a particularly colorful ancient quote about the Gauls, a Celtic group that lived in what is now France.

The Roman soldier and historian Ammianus Marcellinus (who lived circa A.D. 330 to 395) wrote in his book "Res Gestae" ("Achievements") that "a whole group of foreigners cannot stand against one of them in a quarrel if he involves his wife, stronger than him by far and flashing-eyed — least of all when she swells her neck and gnashes her teeth and, swinging her vast, white arms, proceeds to rain down punches mixed with kicks, like shots discharged from the twisted cords of a catapult" (translation by Gavin Kelly).

In other words, don't mess with a Gallic wife.

But we have to be careful with the Ammianus quote as it refers to brawling rather than warfare, said Gavin Kelly, a professor of Latin literature and Roman history at the University of Edinburgh. We shouldn't take the quote to mean that Celtic women fought in battle, Kelly said. He doesn't know if Celtic women fought in battle but, if they did, Kelly suspects it was rare.

Weapons in burials

Rachel Pope, an archaeology professor at the University of Liverpool who has studied the Celts extensively, told Live Science in an email that "we have good evidence now across Europe that a small number of women were buried with weapons." In a 2021 Journal of Archaeological Research study, she mentioned cases in which women were buried with daggers or spears, including a 2,500-year-old cemetery in eastern France where some women were buried with daggers.

Some of the best evidence is from Britain, where "we have osteological evidence from three hillforts now that [some] women took part in combat," Pope said, noting that there "is evidence too for women fighters in northern Britain, where women are buried with chariots."

Simon Mays, a senior human skeletal biologist at Historic England who has studied a woman buried with weapons on the Isles of Scilly, off the southwest coast of Britain, said women in the region were probably involved in battle. There "is little doubt that violence was a routine part of Iron Age life, and [thus] female involvement is likely," Mays told Live Science in an email.

However, just because someone fought in battle doesn't make them a "warrior," Emmet Taylor, a lecturer at the School of Irish Learning at University College Cork, told Live Science in an email. Taylor noted that it "is important for us to think about martial identities complexly," and in ancient and early medieval Ireland, there is little evidence that women were allowed to join social groups that could be considered warriors.

Can ancient Britons be considered Celts?

Most famously, there is Boudica (sometimes spelled Boudicca), a queen of the Iceni (a people in England) who led a revolt against the Roman Empire around A.D. 60. She led her forces to several victories against the Romans before being defeated at the Battle of Watling Street in A.D. 61, after which she died — either by illness or by taking poison to avoid being captured by the Romans.

However, whether the Iceni or other ancient groups in Britain were Celtic is a matter of debate among scholars. The ancient geographer Strabo (who lived circa 64 B.C. to A.D. 24) actually "distinguished Britons from Celts," John Collis, a professor emeritus of archaeology at the University of Sheffield, wrote in his book "The Celts: Origins, Myths and Inventions" (Tempus, 2003).

Terms such as "Celt" and "Gaul" were "never used for the inhabitants of the British Isles except in the most general way for all the inhabitants of western Europe including non Indo-European speakers such as Basques," Collis wrote.

However, although ancient writers didn't refer to people in Britain as Celts, they did note their cultural and linguistic similarities, Freeman wrote. Following this line of thought, it's not too far-fetched to call Boudica a Celt.

Miranda Aldhouse-Green, a professor emeritus of archaeology at Cardiff University in the U.K., told Live Science in an email that she prefers "to refer to Boudica as British rather than Celtic" but noted that Boudica's rebellion is part of substantial evidence for warrior women in Britain.

"There is archaeological evidence for war-women in Britain in the form of Iron Age chariot [burials]," she said. For example, she noted the story of Cartimandua of the Brigantes in Yorkshire, who allied herself with Rome and fought against anti-Roman groups in the region.

"So yes, women did fight particularly in Britain," Aldhouse-Green said. But there is "less evidence" on the European continent for women fighting in battle, she noted.

In Britain the evidence for women in battle is very strong. "In the texts, we have Boudica, raising an army and razing three cities, and [the ancient writer] Tacitus tells us women commanding combat was not unusual," Pope said.

See how much you know about the celts with our Celtic quiz!

Cats are known for the whiskers on their faces. However, if you look closely, you can see that they also have whiskers on their legs. So why do cats have leg whiskers?

To answer why they're useful, it's important to understand what whiskers are. Technically known as vibrissae, whiskers are specialized hairs that are stiffer and several times thicker than other hairs. Although whiskers themselves do not contain nerves, their bases are rooted in specialized receptacles loaded with nerve endings, making whiskers very sensitive.

"Facial whiskers are as sensitive as human fingertips," Robyn Grant, a sensory biologist at Manchester Metropolitan University in the U.K., told Live Science.

A cat's whiskers around the nose, which are also known as the mystacial vibrissae, are the longest whiskers they have, according to the veterinary reference book "The Feline Patient" (Wiley, 2025). Although there is variation among cats, on average there are 12 mystacial whiskers set in four rows on each side of the muzzle.

Facial whiskers help cats hunt when light is limited or when their vision is poor. Although cats are good at seeing distant objects, "cats can't really focus on anything less than 10 inches (25 centimeters) from their eyes, so they use their whiskers to figure out where things are," Jonathan Losos, an evolutionary biologist at Washington University in St. Louis and author of "The Cat's Meow: How Cats Evolved from the Savanna to Your Sofa" (Viking, 2023), told Live Science.

In addition, cats have a tuft of whiskers known as superciliary vibrissae above each eye. These whiskers help protect the cat's vision by triggering a blink when stimulated by anything that might fly into their eye, according to The Feline Patient.

Cats also have chin whiskers (mandibular vibrissae) and whiskers on each side of the head, between the ear and chin (genal vibrissae). These also help cats stay aware of the space around them.

Leg whiskers

Cats also possess a tuft of four to six whiskers on the back of each front leg, each about 0.8 to 1.2 inches (2 to 3 centimeters) long. These are known as carpal vibrissae.

"Carpal refers to the bones that, in humans, are in our wrists," Losos said. "Carpal whiskers are attached to cat forelegs near the carpal bones." However, in cats, the carpal bones are not located in the same place as the human wrist. Rather, they "are further up the leg, because cats essentially walk on their toes and not their palms," he explained.

Cats are not the only animals with carpal whiskers. "Most carnivores have them, and many rodents do, as do lemurs and armadillos," Losos said. "Dogs don't have them, but other canine species do. Rabbits don't have them either, nor do ungulates — that is, hooved animals, such as horses and cattle."

The species that have carpal whiskers "tend to be the ones that either grasp things with their forelimbs or climb up trees," Losos said. "The idea is that if it's hard for the animals to see what they're grasping, carpal whiskers can help supplement their vision. Perhaps cats have them and dogs don't because cats can move their forearms laterally from side to side and put their palms together to grab things, whereas dogs can't."

It remains uncertain what role carpal whiskers play in cats, but they likely "guide locomotion — that is, safe forepaw placement — especially during complex locomotion, such as climbing," Grant said. "They might guide prey handling too."

How much of a cat fan are you? Find out by taking our cat quiz!

Redheads are a rare breed, with people who sport ginger locks making up less than 2% of the global population. With such low numbers, would it be possible for red hair to go "extinct," leaving people with natural hair hues ranging from black to blond, with nary a hint of red?

In a word, no. Experts told Live Science about how the genetics of red hair means the trait is sticking around.

"There is no evidence that redheads are going extinct," said Katerina Zorina-Lichtenwalter, a research associate at the University of Colorado Boulder's Institute for Behavioral Genetics who has studied red hair inheritance. She told Live Science that the genes responsible for red hair "are harmless, they do not interfere with reproduction and they are not linked to a disease that would prevent people from reaching reproductive age."

Nevertheless, the myth of an impending red hair extinction isn't a new one.

"The 'Is red hair becoming extinct?' question has been around for decades," Ian Jackson, a professor emeritus at the University of Edinburgh's Institute of Genetics and Cancer, told Live Science. "There have also been extinction stories about blond hair that go back to the mid-19th century."

The logic behind the supposed redhead extinction usually goes something like this: Red hair is rare; in the world's redhead hotspots in the United Kingdom and Ireland, only about 10% of the population is categorized as redheads. And red hair is a recessive trait, meaning two redhead carriers have to have get together to have redheaded children.

There's even the question of whether potentially deadly conditions, such as melanoma, might wipe out redheads, who are especially susceptible to sunburn. While there are people of color with red hair (Malcolm X famously had reddish hair), red hair is often linked with pale skin and, therefore, elevated skin-cancer risk. Redheads' higher melanoma risk isn't only because they burn more easily. It's also due to harmful chemistry from pheomelanin, the pigment that helps create the reddish hair hue, and reduced DNA‑repair protection, which is linked to variants of the gene that causes red hair.

Keeping the flame alive

But these issues won't doom redheads to extinction, experts said.

First, let's look at genetics. A person needs to inherit the recessive version of MC1R — a gene involved in pigment production — from both parents. Someone with only one copy may not have red hair at all, but they can still pass the variant to their children. So, the role of carrying red hair genes through the generations is shared by people of all hair colors, not just redheads.

This hidden-carrier effect is why a recessive trait can remain in a population even when it is uncommon to see. Jackson gave a simple example: If a variant is carried by half the population, only about a quarter of couples would both carry it. Among those couples, roughly one-quarter of children would inherit two copies.

For red hair to truly disappear, the relevant variants would have to be steadily removed from the gene pool. In evolutionary terms, that would usually mean people carrying these variants — or people with red hair — were having fewer children than everyone else.

"The only reason things go extinct in large populations is because you don't produce as many kids as the others, to put it in blunt terms, and I don't know that there's any evidence for that," Jonathan Rees, professor emeritus of dermatology at the University of Edinburgh, told Live Science.

Second, let's look at skin cancer risk. Rees said people with MC1R variants have a greater skin-cancer risk "somewhere in the region of two to four fold."

But skin cancer risk does not automatically equal evolutionary decline. Many skin cancers occur after people have already had children. Mortality rates for melanoma in the U.K. are the highest in people aged 90 and older, according to Cancer Research UK, after they've had more than enough time to pass on their genes.

Theoretically, before the invention of sunscreen and modern medicine, it may have been more likely for redheads to have died of skin cancer before they had children. Given that the trait has persisted, though, are there any evolutionary benefits to having red hair?

Red hair may carry surprising health advantages. For instance, recent research suggested the red‑hair pigment pheomelanin can help regulate excess cellular cysteine, which can be harmful at high concentrations.

"The vitamin D story is the one that people most often peddle," Rees said. The idea being that lighter pigmentation of the skin (and red hair being linked to lighter skin) increases the amount of vitamin D that people can make during the gloomy winters. But he added that the evidence there is lacking, especially because it can be hard to define who, exactly, is a redhead. It's a spectrum: where does red hair end and strawberry blond hair begin? That makes it hard to study without sequencing the genomes of large swathes of the population.

However, the frequency of red hair could still fluctuate. Jackson said the frequency may "go down somewhat as people become more mobile" and have children with partners from regions where MC1R variants are less common. But that would be a change in frequency, not extinction.

As long as MC1R variants persist silently in carriers — and as long as redheads are not having significantly fewer children than everyone else — the world's redheads are not about to disappear.

Beyond familiar particles such as electrons and protons, scientists have discovered a zoo of "quasiparticles" with exotic names, such as magnons, angulons, dropletons and polaritons. But what are quasiparticles, exactly? And given the "quasi" in their name, are they considered real particles?

One way to understand quasiparticles is to explore what a particle is. The standard mental picture of a particle "is of a discrete object, like a ball," Douglas Natelson, a condensed matter physicist at Rice University in Houston, told Live Science. However, this classic image of particles changed with the advent of quantum physics, which revealed that the universe becomes fuzzy at its smallest levels. For example, in 1924, French physicist Louis de Broglie showed that particles such as electrons could also behave as waves ‪—‬ a discovery that earned him a Nobel Prize.

Modern physics suggests that particles are excitations in fields that permeate the entire universe, a bit like a ripple in a pond, Ross McKenzie, a theoretical condensed matter physicist and professor emeritus at the University of Queensland in Australia, told Live Science. Each type of particle has its own corresponding field. For instance, photons of light are ripples of the electromagnetic field, McKenzie noted.

The quantum nature of particles means they can not only fly through an empty vacuum but also ripple through matter. For instance, photons can zip through transparent and translucent materials, and electrons can flow inside wires.

If you can picture a standard particle, such as an electron, as a ripple traveling within a material, you can also envision other kinds of excitations within matter. These other types of ripples are quasiparticles.

For example, think of "the wave" done at a sporting event. "You can watch it propagate around a football stadium; it has a location and a speed," Natelson said. "It only exists within the stadium, though, and it's made up of the collective response of all the interacting fans."

In much the same way, "quasiparticles can only exist within some medium or material, because they are built up from the response of that material's constituents or building blocks," Natelson said; in contrast, particles such as electrons and protons "can exist in free space."

In other words, quasiparticles can't exist in a vacuum and they can't exist on their own. They're reliant on particles acting together and being in a material where they can emerge, just as a wave carried out in a stadium can only exist when there is a group of people there who can perform it.

A philosophical question concerning quasiparticles is whether they are real, McKenzie noted. After all, "quasi" in Latin means "almost."

"Fundamental particles can exist in isolation in a vacuum, and quasiparticles need many interacting particles to exist," McKenzie said. "Still, I would say quasiparticles are, for all intents and purposes, just as real as particles, in how you can detect them and manipulate them."

A zoo of quasiparticles

The concept of quasiparticles originated with theoretical physicist Lev Landau in the 1950s, McKenzie said, and the idea ultimately helped Landau win the Nobel Prize in physics in 1962. Scientists have now proposed the existence of dozens of quasiparticle types, including the following:

• The phonon, which is a quasiparticle of sound — it is the smallest packet of vibrational energy that makes up sound in matter, McKenzie said.
• The electron hole, often simply called a hole, is the positively charged vacancy left behind once an electron has left its original place, McKenzie noted.
• The electron quasiparticle, which essentially consists of an electron and its interactions with its surrounding environment. This combined package means the electron quasiparticle requires more force to move, so it effectively has more mass than a regular electron, McKenzie explained.
• The exciton, a quasiparticle made of an electron and a hole orbiting each other.
• The anyon, a kind of quasiparticle so far only seen in two-dimensional systems, which may carry only a fraction of an electric charge.

"How many kinds of quasiparticles are there?" McKenzie said. "Just like there are an infinite number of possible states of matter, I would say that, in principle, there are an infinite number of types of quasiparticles."

Scientists describe activity in material as quasiparticles "because it simplifies everything dramatically," McKenzie said. In the same vein, Natelson said that "very often the math behind physical phenomena in solids is described well by quasiparticles."

Quasiparticles are not simply useful for bookkeeping. For instance, researchers typically describe the behavior of electricity in electronic devices in forms of quasiparticles such as holes and excitons, Natelson said.

All in all, the answer to the question of whether quasiparticles are real is a matter of interpretation. If the question is whether or not a quasiparticle is an elementary particle such as an electron, the answer is no — they cannot exist on their own, and they depend on the materials in which they manifest.

However, if the question is whether or not quasiparticles are real phenomena, the answer is yes — they are things that scientists can measure that can behave like particles and effectively have many of the same properties that other particles do.

Nowadays, the word "weapon" may evoke modern-day technologies such as firearms, blades and missiles. Yet archaeological evidence shows that weapons have existed for hundreds of thousands of years. Of course, early humans wielded weapons that looked quite different from those used today.

So what is the oldest weapon?

Generally, weapons are defined as objects used to impart violence on other living beings, said Ben Fitzhugh, a professor of anthropology at the University of Washington in Seattle. That includes objects used in interpersonal conflict among hominins (a group that includes modern Homo sapiens and our relatives that evolved following the split from chimps and bonobos) and for hunting, Fitzhugh told Live Science.

But archaeologists don't automatically consider just any object that could cause harm to be a weapon; context and intent matter, Rolf Warming, founding director of the Society for Combat Archaeology and a doctoral candidate in archaeology at Stockholm University, told Live Science in an email. For instance, experts excavating a site with potential weapons might look for evidence of hunting, such as butchered animal bones; or artifacts like pointed bone tools that can be used to craft weapons. They may also consider what objects were made out of and residue or wear marks on the weapons themselves, which can help determine how they were used.

According to Warming, archaeologists generally consider three finds as contenders for the oldest evidence of weaponry in the archaeological record: the Clacton spear, the Schöningen spears and throwing sticks, and the Kathu Pan 1 spear points.

Clacton spear

Scientists have estimated that the Clacton spear is over 400,000 years old. Experts think the uncovered 15-inch-long (37 centimeters) spear point was broken off from a longer shaft, perhaps by accident while the user was hunting, or deliberately as a ritual. It's made from yew wood and was worked with a tool to a sharp point on one end. The broken end was never found.

Excavated in 1911 from Clacton-on-Sea in Essex, England, the Clacton spear is now on display in London's Natural History Museum, where it is labeled as the oldest preserved wooden spear fragment in the world.

Archaeologists believe it was fashioned by either Homo heidelbergensis or Neanderthals — two hominins that predate H. sapiens. In fact, the spear appears to be older than our species, as the oldest known H. sapiens fossils date to around 300,000 years ago. The Clacton spear's shape, size and wood suggest that it was a thrusting weapon and that its makers engaged in cooperative hunting strategies.

Schöningen spears and throwing sticks

Since 1992, archaeologists have excavated between 20 and 25 wooden weapons at a site in Schöningen, Germany, including complete and fragmented spears, and throwing sticks made predominantly from spruce and pine wood.

They're considered the oldest completely preserved wooden weapons in the world, according to the Schöningen Research Museum; today, archaeologists' varying estimates place the weapons' age between 200,000 and 300,000 years old. Their creation has historically been attributed to H. heidelbergensis, an early human species that lived about 700,000 to 200,000 years ago throughout Europe, parts of Africa, and possibly in what is now China. More recent dating suggests the spears could potentially be the work of Neanderthals, according to a 2025 study.

Scientists think these spears, which average just over 6.6 feet (2 meters) long, were used as thrusting weapons, throwing weapons or some combination of both to hunt larger game mammals, like horses, according to a 2024 study. The shorter throwing sticks are believed to be projectile weapons and average around 26 inches (65 centimeters) long.

"In fact, the [Schöningen weapons] were found in association with butchered horses," Fitzhugh noted. The spears were sharpened, hardened and blackened using fire. "These things were shaped in a dynamically sophisticated way," he said.

According to Warming, the Schöningen weapons suggest hominins had clear material knowledge and used different hunting tactics depending how far they were from their target.

Kathu Pan 1 spear points

Hafted spear tips — stone points attached to some form of handle or shaft — became more common in the archaeological record between 200,000 and 300,000 years ago across Africa and Eurasia, and were previously thought to be created by Neanderthals or H. sapiens.

However, 500,000-year-old stone points excavated from the Kathu Pan archaeological site in South Africa have signs of wear on the edges, and the shape of their base is consistent with hafted spears, suggesting they were used as weapons. Their age predates both Neanderthals and H. sapiens, and the location they were found in suggests they could have been created by H. heidelbergensis, who lived in regions of southern Africa at that time. Archeologists think they could have been used as spear ambush weapons.

But some researchers have contested this evidence, arguing that it's "indirect" or even controversial, as no wooden shafts were uncovered with the spear points to solidify them as weapons as opposed to stone tools.

Something older?

These are some of the earliest examples of weapons in the archaeological record, but there may be older weapons that have not survived throughout time.

While hominins likely used organic materials like wood, sinew, fiber and adhesives to create objects and weapons, these materials break down or are destroyed over time, Warming noted.

Studying these ancient weapons gives us perspective about the present. After all, weapons are more than tools of harm: Warming said they are material clues that give us glimpses into how our early relatives combined materials, used their imaginations, shared knowledge and cooperated in groups.

Yet among these three important finds, Warming said that when inquiring about the oldest known weapon in the world, that "Clacton is the safest answer."

Can you identify these historical objects of war? Test your smarts with our weapons of the world quiz!

Animals come in a wide range of shapes and sizes, from spindly stick insects to blubbery whales. Round animals are harder to come by, but they're more common in the ocean than on land. So what is the roundest animal, and why are round animals uncommon?

At a glance, round animals might not seem that out of place on land. After all, small mammals like rabbits and pikas often look a bit spherical when their fur is puffed out and they're sitting. But Chris Law, an evolutionary biologist at the University of Washington in Seattle, said this shape is mostly an illusion.

"They're physically not as round as they appear to be," he told Live Science.

A sphere has the lowest surface-area-to-volume ratio of any shape, which is ideal for heat conservation. Law said the ability to curl or scrunch into a sphere can be essential to the survival of small animals, which lose heat more quickly than larger animals do.

Rolling into a circle can also offer protection. To safeguard their vital organs, animals like armadillos and hedgehogs roll into balls when threatened, leaving only their armor or spikes exposed to predators. But when you look at the skeletons of these supposedly round critters and their shape when they're not scrunched up, they aren't round at all, Law said.

In fact, there are several reasons why round creatures are rarely found on land. Being perfectly round all the time would make it difficult to fit into small spaces, which is a key survival technique for avoiding predators, he added, and with the effects of gravity, having a lot of unsupported round mass puts strain on an animal's body. Overly round mammals, like obese cats and dogs, often suffer joint issues and other health problems.

"As a round animal, you're probably going to lose mobility, and you're going to be picked off by predators pretty easily," Law told Live Science.

There are exceptions, however. For example, beetles are small enough not to be affected as much by gravity and can fit into nooks and crannies without much wiggling. Their round shape coupled with their hard carapace makes them more difficult for predators to eat. And rain frogs puff up into a round shape to keep predators from removing them from their hiding spots.

If we're counting animals that are only round sometimes, then pill bugs (also known as rolly pollies) might take first place for the roundest land animal. But if not, rain frogs, and beetles like ladybugs, are the top contenders for the roundest land animal. But even these animals aren't perfectly round; ladybugs have flat bottoms to make it easier to move around, and rain frogs are less spherical when not puffed up.

Round sea animals

Being round at sea is a little more feasible than it is on land. In fact, sometimes, it's critical to survival.

Karly E. Cohen, a biomechanist at Friday Harbor Labs in Washington, studies lumpsuckers, a type of fish she describes as "quite round." These bulbous fish have suction cups made of enamel on their stomachs, which they use to latch on to surfaces at the bottom of the ocean. Lumpsuckers have armor, also made of enamel, covering their round shape. According to Cohen, lumpsuckers' roundness plays a huge part in keeping them attached to the seafloor or other surface. The curve of their bodies, coupled with the tooth armor, modifies the drag around the animal, creating a force that pushes them down rather than shooting them into the water column.

Although gravity still works underwater, buoyancy helps push objects up, meaning the effects of gravity do not feel as strong at sea as they do on land. According to Cohen, lumpsuckers can get around just fine, if a bit more slowly than their more streamlined neighbors. And like pufferfish and porcupine fish, lumpsuckers' round shape and uninviting exterior make them tough to swallow ‪—‬ literally.

"There's no real good way to eat an apple whole; you have to take a bite," Cohen told Live Science. "And then this thing is armored, so it's very hard to take a bite."

All about symmetry

Despite their spherical shape, lumpsuckers don't quite win the award for the roundest animal. Lumpsuckers, like most animals, have bilateral symmetry, which means they are divided into two symmetrical sides across one plane of symmetry. But there are animals that have radial symmetry, which means they are symmetrically arranged in a circle around a central point with multiple planes of symmetry.

Echinoderms ‪—‬ which include sea stars, sea urchins and sand dollars ‪—‬ have pentaradial symmetry, which means they have five planes of symmetry arranged around a central point. According to Laurent Formery, a developmental biologist at the Oceanological Observatory of Banyuls-sur-Mer in France who studies echinoderms, scientists aren't entirely sure of the benefits of pentaradial symmetry or why echinoderms are the only modern animals that have it.

However, Formery thinks being divided into five sections instead of two may give echinoderms better sensory perception than bilaterally symmetrical animals. Echinoderms have decentralized nervous systems, meaning that instead of a brain, they have a series of neurons distributed evenly across their body, and photoreceptors all over their skin.

"They are kind of like a big crawling eye and brain, so they are receiving information from everywhere," he told Live Science. "They have a completely different way, that is difficult for us to imagine, of interacting with their environment."

Because of this unique organization, radially symmetrical animals can sense predators from any direction. They can also easily switch which segment of their body is in the lead, making it easier for them to change directions to capture prey.

Discounting their spikes, certain species of sea urchin are almost perfectly spherical. Like lumpsuckers, sea urchins' spherical shape, coupled with their spines, makes them difficult meals for predators, Formery said. Sponges lack any symmetry, so they can develop all sorts of body shapes. One species, called the death ball sponge, looks like a Sputnik lamp, with many hook-covered spheres attached to its spindly "arms."'

Under the sea, urchins in the genus Histocidaris, like Histocidaris purpurata and Histocidaris formosa, are the closest to perfect spheres, making them the top candidate for roundest animal.

Although roundness might be uncommon in the animal kingdom, Cohen said it's fascinating to study these animals and the reasons for their adaptations.

"As somebody that's interested in the tools and ways organisms work in their environments," she said, "they're a real treasure trove of evolution."

What do you know about the animal kingdom? Test your knowledge with our animal quiz!

Albert Einstein gave us the theory of relativity, explained the photoelectric effect and predicted the existence of gravitational waves. He revolutionized our understanding of space, time and gravity.

In fact, he is so famous for his brilliance that "Einstein" is synonymous with "genius." But being a genius doesn't mean you're infallible. So was Einstein wrong about anything?

"For sure, he was wrong about tons of things," Nicolás Yunes, a theoretical physicist at the University of Illinois Urbana-Champaign, told Live Science. "We remember him for the things he was right about, for the most part, because the things he was right about shocked the scientific world and eventually those ripples affected everyone on Earth."

For example, one of his most famous predictions was marred by a mathematical mistake. In 1916, Einstein correctly realized that the acceleration of matter could produce ripples in space-time that we now call gravitational waves. But when he and the physicist Nathan Rosen described these waves using mathematics, they ran into a problem: Any solution to the equations of general relativity that would allow for gravitational waves "blew up," Yunes said. "They had these singularities … these divergences. And solutions that diverge and explode like that, they can't be physical representations of reality."

As a result, Einstein changed his mind and decided that gravitational waves couldn't exist. He wrote up his results and submitted them to the journal Physical Review, which had recently begun sending papers to outside experts for peer review. An anonymous reviewer caught an error in Einstein's math, and when Einstein learned about it, he was so furious that he withdrew the paper and submitted it to a different journal.

But the reviewer had spotted a real error: Einstein's mathematical infinities were a coordinate artifact. Much like Earth's lines of longitude seem to converge to a "singularity" at the North Pole even though nothing unusual is happening there on the ground, Einstein's math could have been fixed by using a different set of coordinates.

Without Einstein's knowledge, the reviewer befriended and demonstrated the error to Einstein's assistant, who explained it to Einstein. Einstein corrected the error and republished the paper with the opposite conclusion, showing that gravitational waves do, in fact, exist.

Calculating "impossible infinities"

Remarkably, Einstein stumbled on the same error twice — at least, it seemed that way. When examining the mathematics around black holes, he once again calculated impossible infinities ‪—‬ this time, at the edge of a black hole. And once again, he concluded that this must mean the object couldn't exist.

"Einstein remained skeptical of the existence of black holes," John D. Norton, a professor in the Department of History and Philosophy of Science at the University of Pittsburgh, told Live Science in an email. "He was resolute in his judgment that there would be a singularity in space-time already at the event horizon of a black hole, which is now merely regarded as marking the point of no return for those falling into a black hole."

But this time, Einstein was not convinced that he made a mistake. Norton argues this wasn't just stubbornness; it reflected a philosophical belief in how Einstein thought mathematics and physics should relate to each other.

"He was unmoved by alternative analyses that found his mathematical infinities to be merely artifacts of the particular mathematical methods he preferred," Norton said.

Einstein and quantum mechanics

Einstein's resistance to quantum mechanics is probably his most famous wrong turn. His central objection was to a phenomenon called quantum entanglement: the idea that two particles can be linked in such a way that measuring one instantly affects the other, no matter the distance between them. In a 1947 letter to his colleague Max Born, he wrote, "I cannot seriously believe in it because the theory cannot be reconciled with the idea that physics should represent a reality in time and space, free from spooky actions at a distance."

Einstein thought this instantaneous phenomenon seemed to violate special relativity, which says that nothing can travel faster than light. As a result, he believed quantum mechanics must be incomplete and that there was some deeper, unknown description of reality that would restore order.

"He died not liking quantum mechanics," Yunes said. It wasn't until 1964, almost a decade after Einstein's death, that John Bell proved entanglement was real.

"Today, a lot of the technology that we have relies on quantum mechanics, and so we know it's correct," Yunes said. "But it's still incompatible with general relativity, with [Einstein's] classical theory."

"It's possible that general relativity is wrong," Yunes said. "It's also possible that quantum mechanics is not the right description when you're talking about systems that are very strongly gravitating and you're looking at Planck-scale-type dynamics," which are incredibly small scales at which quantum effects dominate. For instance, the center of a black hole, which is both compressed to the quantum scale but also sits inside the most intense gravitational field in the universe, is a place where general relativity predicts a singularity that quantum mechanics has no way to describe.

But many of Einstein's mistakes still moved science forward. "General relativity is a good example," Norton said. "Einstein based it on the idea that he was generalizing the principle of relativity to acceleration and on what he soon called Mach's principle. Neither proved compatible with his final general theory of relativity."

Einstein's mistakes might come as a surprise to others, but they didn't to him. Once, when writing a book with his collaborator Leopold Infeld, Infeld told him he was taking special care with it because Einstein's name would appear on it. Einstein laughed and said, "There are incorrect papers under my name, too."

See how much you know about Albert Einstein with our Einstein quiz!

Fast charging seems to be almost everywhere. Many smartphones can go from nearly empty to more than 50% charged in about half an hour, while some electric vehicles can add hundreds of miles of range during a quick charging stop.

But batteries aren't perfect; their capacity degrades over time. Given that fast charging delivers more power in a shorter amount of time, does fast charging damage batteries?

Scientists say the answer is yes, but it's more complicated than you might think. Fast charging can accelerate some types of battery degradation, but modern batteries are designed with safeguards to help limit the damage.

How does fast charging work?

Rechargeable lithium-ion batteries — the most common battery type in the world — work by moving lithium ions between two electrodes called a cathode and an anode. During charging, lithium ions travel through the battery and are stored in the anode until the battery is used again.

The main difference between fast charging and regular charging is how quickly those ions move. Compared with regular charging, which can take hours, fast charging can refill a battery in an hour or less.

"Regular charging applies a lower current, allowing lithium ions to intercalate [move into microscopic holes] into the anode gradually, which generates little heat and causes minimal mechanical stress," Zhiyuan Jiang, an associate professor in the Department of Chemical Engineering and Technology at Xi'an Jiaotong University in China, told Live Science via email. "Fast charging increases the current [and] power significantly to shorten charging time."

Not all batteries are designed for fast charging. A battery's ability to handle high charging speeds depends on its materials, internal structure and battery management system, Jiang explained. Fast-charging batteries often use specialized electrode materials or thinner electrodes and electrolytes that allow the lithium ions to move more easily. Manufacturers may also redesign the battery's internal architecture to reduce resistance and heat buildup.

Stanislaw Zankowski, a battery researcher at the University of Oxford, compared the process to traffic moving through a city.

"You could think about charging a battery as transporting people through roads, intersections and buildings," Zankowski told Live Science. "Fast charging is really a question of how efficiently you can move all that traffic without creating bottlenecks."

What type of damage could fast charging cause?

All lithium-ion batteries lose capacity over time, even when they are treated carefully. But fast charging can speed up some of the chemical processes responsible for that aging.

One of the biggest concerns is a process called lithium plating. During rapid charging, lithium ions may not have enough time to settle properly inside the anode. Instead, some lithium can accumulate as metallic deposits on the electrode's surface. These deposits can reduce the amount of lithium available to store energy, thereby lowering the battery's capacity. In extreme cases, the lithium can form needle-like structures called dendrites that puncture internal battery components and create safety hazards.

Fast charging can also generate more heat. Heat is a natural byproduct of electrical resistance in the battery. The faster a battery charges, the more heat it produces.

"For charging a small battery with a small current, that amount of heat will be also relatively small," Zankowski said. "So, it's not really a safety problem, but as we increase the size of the battery, the amount of current that we'll be pushing during charging and the amount of heat will also increase quite a lot. And as a result, we can't really charge larger batteries as quickly, mostly because of the safety margin."

Higher temperatures can accelerate chemical reactions that gradually degrade battery materials. In some extreme cases, overheating can cause batteries to swell, catch fire or even explode ‪—‬ a process known as thermal runaway.

Fortunately, most modern smartphones, laptops and electric vehicles contain sophisticated battery management systems that monitor voltage, current and temperature during charging. That's why you may get a heat warning from your smartphone if you leave it in the sun. If temperatures climb too high, these systems automatically slow charging to protect the battery.

Best tips for battery life

So what's the best way to protect a battery while charging it quickly?

Both Zankowski and Jiang emphasized that temperature is key. It's best to avoid charging devices in hot environments, such as inside a parked car or in direct sunlight. Extremely cold temperatures can also be harmful because they make it harder for the lithium ions to move through the battery.

"Ideally, the temperature range should be around 20 to 25 degrees Celsius [68 to 77 degrees Fahrenheit] for charging," Zankowski said. "So, just like a comfortable temperature for a human being, right?"

Experts also recommend that you avoid keeping devices like laptops constantly plugged in, as this can degrade battery performance. Jiang suggested implementing the "shallow charge, shallow discharge" habit.

"Keep your battery between 20% and 80% for daily use," he said. "It is not necessary to charge to 100% every time."

Can you match these ancient devices to their pictures? Find out with our computing quiz!

With violent conflicts happening across the world right now, war feels like a permanent condition. But have humans ever lived without war?

The answer depends on how you define "war." If war means a fight between two governments, then yes, there have been peaceful periods "because for nearly 99% of human history, there were no governments," said Ian Morris, a historian at Stanford University and author of "War! What Is It Good For? Conflict and the Progress of Civilization from Primates to Robots" (Farrar, Straus and Giroux, 2014).

Violence, on the other hand, has always been with us. If the question becomes "Have people ever lived without violence?" then "the answer is pretty clearly no — people have always fought and killed each other," Morris told Live Science in an email.

The theory of a peaceful prehistory

According to a review of studies published in 2022, war was rare or nonexistent in early human history when people lived as nomadic hunter gatherers. Peter Stearns, a professor emeritus of history at George Mason University and author of "Peace in World History" (Routledge, 2014), agrees with this idea. There was "little or no war in hunting and gathering [cultures] before the rise of agriculture," he told Live Science in an email.

This idea is based on archaeological records. Researchers examining ancient human bones from around the world have looked for skeletal evidence of war injuries, for instance of multiple individuals with unhealed wounds from stabbing, slicing and blunt force trauma who were buried in mass graves. However, they found little evidence of war injuries before 8000 B.C. After that, when humans began transitioning from nomadic life to permanent settlements, such injuries appeared widely.

This doesn't mean that lethal violence was absent in prehistoric societies. Researchers in Kenya found 27 skeletons dating to 10,000 years ago at the archaeological site of Nataruk; these skeletons show signs of violent death, which some anthropologists interpret as evidence of intergroup violence among early hunter-gatherers. And at Jebel Sahaba, a prehistoric cemetery in Sudan, archaeologists unearthed 13,000-year-old remains bearing signs of intergroup attacks.

But these cases of interpersonal violence don't count as war, because of how researchers usually define it. "Researchers who focus on war as a specific category of violence usually define it by saying it has to be violence organized by a government or else collective violence that kills more than a certain number of people," Morris said. "Prehistoric societies rarely had formal governments and hunter-gatherer bands rarely had more than a few dozen members, so if you define war as a conflict run by a government or one that kills >100 people, then by definition there can't have been wars in [early] prehistory."

David Christian, a historian and professor emeritus in the Department of History and Archaeology at Macquarie University in Australia, echoed this idea. "For much of human history communities were so small that it is not clear if we can equate violence with war," he told Live Science in an email. "I guess we can say that humans have always been capable of violence and as communities got larger that violence began to take on forms that we might want to describe as 'war.'"

Peace between rival powers

Once large kingdoms and empires appeared, war became very common. A doctoral thesis written by Jared Morgan McKinney, now an assistant professor of international security studies at Air War College in Alabama, focused on periods of peace between major powers. He concluded that war was basically the norm throughout history and that famous "peaceful" eras, like the Pax Romana (Roman Peace), usually just meant one powerful group had beaten everyone else into submission.

But there were exceptions. "Wars are expensive and risky," said Peter Frankopan, a professor of global history at the University of Oxford. "So in many periods in history, stability and peace have been achieved by rivals, adversaries and neighbors being able to match each other's capabilities."

McKinney's thesis identified several periods when rival powers managed to avoid war.

From around 1400 to 1200 B.C., the big powers of the ancient Middle East — mainly Egypt and the Hittite Empire, an ancient civilization in modern-day Turkey — went through two unusually long stretches without major wars. Peace was possible because the "Great Kings" recognized one another as equals and settled their territorial and political differences through formal treaties rather than war.

For most of their history, Rome and Persia were at war. Then, from roughly A.D. 387 to around 501, the two superpowers mostly stopped fighting. Historians call this the "Long Fifth Century" and have put forward many explanations for this period of peace, including that both sides faced serious outside threats that made fighting less affordable and that the two sides developed a language of "brotherhood" that acknowledged each other as equals rather than enemies, McKinney wrote in his thesis.

Between roughly A.D. 1000 and 1200, the rich Song dynasty in China secured peace with its militant northern neighbors ‪—‬ the Liao and the Jin‪ —‬ by paying them regularly to keep the peace. Although these monetary gifts (or bribes) looked like weakness, the payments were tiny compared with what China was earning through trade, according to McKinney.

Another example, according to Morris, is the "long peace" between China, Korea and Japan between about 1600 and 1850. While European states spent those centuries competing for power and territory through war, the East Asian states lived in relative peace. "In Europe, we've tended to be a lot more aggressive and competitive — which gives the impression that war is a 'natural state' of being," Frankopan said.

A notable period of relative peace in North America was the Long Peace among the Iroquois nations, McKinney told Live Science in an email. For roughly three centuries, from about 1450 to 1777, five (and later six) Native American nations who had previously engaged in violent and costly conflict forged a peaceful relationship known as the "Haudenosaunee Confederacy" or "League of Five Nations."

In South America, McKinney pointed to the South American Long Peace. This refers to the absence of major interstate wars between sovereign nations in South America ever since 1935.

"War is 'normal' in history," McKinney said. But, as these examples show, "patterns have exceptions."

Can you identify these historical objects of war? Test your smarts with our weapons of the world quiz.

When you click to enter a website or try to log in or fill out a form, you may be asked to identify motorcycles from a grid of grainy images, decipher a string of convoluted characters, or click a box that states "I am not a robot."

These tests are called CAPTCHAs, which stands for "Completely Automated Public Turing test to tell Computers and Humans Apart." As their name suggests, they are meant to help a website distinguish if an action is coming from a human or a bot, since the aforementioned tasks are theoretically easy for a human and difficult for automated software to perform. This, in turn, blocks bots from spamming comments, downloading files, taking over accounts, or executing any other action on a website.

But as computer models increasingly gain the ability to solve CAPTCHAs, thanks to advancing artificial intelligence (AI) — and with puzzles getting weirder and more difficult for humans to complete — does this mean CAPTCHAs are still useful?

CAPTCHAs were introduced in the late 1990s to address "a very simple, but very difficult problem," Andreas Plesner, a computer scientist at ETH Zurich, told Live Science. "If I don't interact with a person physically … is it a computer? Or is it human?" Some of the first CAPTCHAs, still common on websites today, were composed of distorted text, since text-reading software at that time had trouble interpreting warped words.

But over time, text-reading software improved and new types of CAPTCHAs were developed. For instance, reCAPTCHA, one of the most popular CAPTCHA services, has an image-based test that asks users to identify objects such as traffic lights, motorcycles or bicycles from a grid of Google Street View photos. This was developed after Google acquired the service in 2009.

"The bet was that recognizing objects in messy, real-world photos was still a uniquely human skill," Ng Chong, chief of information technology and director of United Nations University's Campus Computing Centre in Tokyo, told Live Science in an email.

As time went on, CAPTCHA design continued to advance. In 2014, Google came out with reCAPTCHA v2, which analyzed computer mouse behavior by asking people to click a checkbox to test if a user was human. If the behavior is deemed suspicious, determined by factors like how a user interacts with the site beforehand or the timing of their click, the street-image grid pops up as an additional puzzle.

However, more recently, technology has improved to a point where image recognition is no longer a human-specific skill. As early as 2016, researchers found that low-cost deep learning technologies could solve reCAPTCHAv2 around 70% of the time. By 2024, Plesner and his colleagues developed an AI model that could solve the puzzles correctly 100% of the time. Earlier in 2026, Chong noted that he built a tool that could mimic human-like browsing behavior and sometimes bypass reCAPTCHA v2 without triggering the image grid at all. When the grid was triggered, the tool used AI to solve it within a few tries.

"When both the challenge and the behavioral layer are defeated by commodity tools running on a single laptop, the fundamental premise of CAPTCHA, that there are tasks humans can do but machines can't, stops holding," Chong wrote.

Looking ahead

So does that mean CAPTCHAs are completely obsolete? Not quite. Although the model Plesner and his colleagues developed breezed past reCAPTCHAv2, "there were a lot of the safety measures that were not tied to being able to solve it, but more tied to how you solve it," he said. For example, while conducting their research, Plesner noted that his team used a virtual private network (VPN) that changed IP addresses for each test, since a single IP address with a high volume of solved CAPTCHAs faced tasks with increasing difficulty, or got blocked entirely.

Modern CAPTCHAs focus on these background clues and tactics, rather than the puzzle itself. This includes Google's reCAPTCHA v3, Friendly CAPTCHA, hCAPTCHA and Cloudflare's Turnstile, among others, which run without sending a puzzle at all. They instead look at whether the action is coming from a real attested device (rather than from automated code), whether an IP address has had a high volume of automated requests in the past, how a user navigates a webpage, what the user's cookie history is, and a slew of other factors to determine possible malicious intent.

As the tug-of-war continues, CAPTCHA puzzles are still widespread. After all, they’ve been the status quo for decades, are easy to set up and are relatively cost-effective, Chong said. But these tasks have some other drawbacks. Although bots can increasingly solve the puzzles with ease, CAPTCHAs can be a headache to get through for humans and can be seen as discriminatory against those with disabilities, notably visual disabilities, as a researcher noted in a 2022 conference paper.

The crescendoing complexity of CAPTCHA puzzles has even been the subject of parody, with developer Neal Agarwal creating a free satirical game called "I'm Not a Robot." Users must solve a series of increasingly convoluted verification checks — scoring a point for each stage they pass, which eventually transcend into the absurd.

So, as machines get smarter, the answer may not be to find more difficult puzzles. "If a CAPTCHA can only be solved by someone with a Ph.D. in mathematics, then it's not very useful," Plesner said. "The internet needs to be used by everyone."

Can you match these ancient devices to their pictures? Find out with our computing quiz!

If you push two metal plates together on Earth, nothing happens. But if you take those same plates into the vacuum of space, they can fuse into a single piece of metal.

This phenomenon, called cold welding, has been a known hazard for spacecraft engineers for a long time. So what's actually happening at the atomic level, and why does space make it so much easier?

The answer comes down to a lack of oxygen in space, experts told Live Science.

Metals are made of lattices — structures where atoms are bonded to one another. But atoms near the surface of a metal aren't bonded to anything on the outward-facing side. If given the chance, they'll "reach out" and share electrons with the surface of another piece of metal.

But on Earth, nearly every metal surface is coated in an oxide layer. The layer is just a few atoms thick and forms when metal meets oxygen. "Once the oxide is formed, it's over," Julia Greer, a materials scientist at Caltech, told Live Science. "Then it can't cold weld anymore, because the oxygen basically passivates these bonds."

That thin oxide acts like an insulating wrapper. Without it, the free electrons at the surface of one metal piece stop recognizing which atom they belong to. "Those electrons don't know if it's in this piece or if it's in that piece, so they begin sharing the electrons, and essentially that cold welds things together," Sven Bilén, a professor of engineering design and aerospace engineering at Penn State, told Live Science.

In space, there's no oxygen to rebuild that layer once it's gone. The cold and the radiation make things worse. Bombardment from solar and ionic radiation in orbit can scour metal surfaces clean, Greer said, leaving freshly exposed atoms primed to bond. "Everything in space is conducive to cold welding," she said.

Metal surfaces are never perfectly smooth, either. At a microscopic level, they're jagged ‪—‬ more like tiny mountain ranges than flat plains, said Zachary Cordero, an aerospace engineer at MIT.

Pressing two surfaces together, especially with any sliding or vibration, can shear off the oxide layer that formed while the metal was on Earth and flatten those peaks into metal-to-metal contact. "You're breaking up the surface oxide, and you're forming metallurgical bonds," Cordero said.

Why cold welding worried early spacecraft engineers

Cold welding in space has long been a problem. "If there is cold welding, things can become stuck in place," Cordero told Live Science. "If you have a deployable structure and there's cold welding, you might freeze the mechanism, or a door might become locked, or something might become immobilized, which you don't want."

For example, say you were to add a metal screw to a metal door. After a while, you would not be able to unscrew it because it would have become part of the door.

Bilén pointed to NASA's Galileo probe, which launched in 1989: lubricant loss and launch vibrations during launch are thought to have stripped the oxide layer from parts of its furled high-gain antenna. When engineers attempted to deploy the antenna in 1991, it never fully opened.

Some metals are more troublesome than others. Gold and platinum don't form an oxide layer at all, even on Earth, which makes them notoriously prone to cold welding. "Gold definitely is a very notorious metal for cold welding," Greer said, adding that gold's softness lets it conform easily to whatever surface it touches and it bonds even more easily.

How to prevent cold welding in space

To prevent components from accidentally fusing in orbit, engineers rely on a few strategies. One is anodizing, a process that locks an artificial oxide layer onto a metal surface. Another method is to coat moving parts with dry lubricants, such as molybdenum disulfide, to physically keep surfaces from touching.

A third strategy is to pair dissimilar metals — for instance, gold next to a "body-centered" metal, like molybdenum — so their atomic structures don't mesh as neatly. "Their packing order is not quite perfectly aligned, and so there'll be a lot more energetic kind of barrier to overcome," Greer said.

Before launch, hardware also gets shaken on vibration tables and cycled through extreme hot-and-cold swings inside vacuum chambers, simulating the stresses of liftoff and orbit to catch problems on the ground.

Even with all of those precautions, cold welding can still happen. Bilén recalled bolts in his own lab's vacuum chamber fusing shut after a move across campus. They eventually had to be drilled out. "It happens even on Earth," he said.

See how much you know about human exploration into space with our spaceflight quiz!

Ancient Roman transportation engineers built a vast network of roads that stretched across Europe, North Africa and parts of the Middle East. This complex road system was a key to travel and trade in the Roman Empire, and many of the centuries-old streets had a reputation for being extremely straight, although not all of them were.

For instance, Via Appia (Appian Way), which connected Rome to the port of Brundisium in southern Italy, was more than 300 miles (500 kilometers) long, and sizable parts of it were straight. Another Roman road, Stane Street in southern England, was built to connect London to Chichester. Much of the road, which stretches roughly 57 miles (92 km), is straight. The Middle East also had straight Roman roads, including a coastal avenue from Antioch, Turkey, to what is now Gaza.

A recent mapping project was able to map about 186,400 miles (300,000 km) of roads and more are probably undiscovered. But how did the Romans avoid unnecessary twists and turns to ensure straight streets? The answer may lie in three surveying instruments the Romans used.

Surveying roads

In some instances, the Romans built on top of older roads that existed before they conquered an area. Their "road network incorporated older roads from a broad range of different societies and polities," Marion Kruse, an associate professor of classics at the University of Cincinnati, told Live Science in an email.

But when the Romans built new roads, they used a few different tools to help plan them.

"Three instruments were used consistently by Roman road builders: the dioptra, the groma, and the chorobatus [or chorobates]," Adriana Panaite, a researcher at the Vasile Pârvan Institute of Archaeology in Romania who has studied Roman roads extensively, told Live Science in an email.

While the dioptra is known from ancient texts, no example of it has ever been found in an archaeological dig, according to M.J.T. Lewis, who was a historian at the University of Hull in the U.K. In his book "Surveying Instruments of Greece and Rome" (Cambridge University Press, 2001), Lewis noted that the design of the dioptra varied considerably. The different designs tended to include a stand and a disc-shaped base with a tube-shaped sighting instrument attached. An ancient surveyor could look through the tube and see a distant object without extraneous light interfering, allowing for a better view.

The chorobatus was used to measure horizontal planes. Around 20 feet (6 meters) long, the chorobatus was a beam of wood on legs and looked like a small table, Lewis wrote. It likely had small weights hanging from it to show that the beam was level. No ancient example of the chorobatus has survived, and the exact design and way it was used are unclear. Ancient texts indicate that it acted as a builder's level that could help set up level points and determine elevations.

But the most important tool Roman surveyors used was the groma, Joseph Lewis, an archaeologist at the University of Cambridge who has conducted extensive research on Roman roads, told Live Science in an email. The "groma was the principal tool of the mensor ‪—‬ the land surveyor ‪—‬ when planning long, straight alignments," he noted. "These alignments were then often used when constructing roads across gentle terrain."

The groma, also called a gruma or croma, consisted of a vertical pole with a horizontal X-shaped cross on top and four small weights hanging down from strings at the ends of the crossbeams, Lewis said. The groma was most useful for establishing right angles.

Multiple Roman surveyors could use the weights on their poles to ensure any road they were building was going in the right direction.

"A surveyor at one end will direct other surveyors to move their poles until they are in line with one another," Lewis explained. "Once the direction had been established, Roman surveyors observed the landscape and adjusted the course of a road to accommodate/avoid obstacles such as steep sections which would have posed difficulties for wheeled vehicles, locations to ford rivers, or to connect pre-existing settlements."

However, the techniques used to build roads throughout the Roman Empire probably varied somewhat, meaning experts "should be cautious about assuming that there was a single 'Roman' technique for road building," Kruse said. He noted that the Roman Empire covered a vast area and lasted for a long period of time.

"It seems safe to assume that practices varied across time and space," Kruse explained.

Not all roads were straight

One reason for the variation in Roman roads was likely the diversity of the laborers who built them. "In all likelihood, road-building work was done by a mix of soldiers, slaves ‪—‬ especially prisoners ‪—‬ and free locals called upon to help as part of a 'corvee' obligation imposed by their local community on Rome's instructions," Richard Talbert, a professor emeritus of history at the University of North Carolina at Chapel Hill, told Live Science in an email. But paid laborers were probably used for some skilled tasks, such as building bridges, Talbert said.

While Roman roads have a reputation for being precisely laid out and straight, not all of them were.

"It is commonly mentioned that Roman roads were exceptionally straight. This is partially true: The Romans had engineers and a large labor force that in some cases managed to modify the landscape to enable straight roads," Tom Brughmans, a classical archaeologist at Aarhus University in Denmark who is part of a team that helped create an updated map of the Roman road system, told Live Science in an email.

"We believe that the Romans preferred relatively straight roads in places where there was very little friction offered by the topography," such as flatland areas, Brughmans said. But in areas with more difficult terrain, such as mountainous regions, the roads would often not run straight.

Brughmans said he expects "future research will show that, in general, [Roman roads] are less straight than modern roads, given the need for motorized vehicles to avoid sharp turns when driving at speed."

From Augustus to Nero, see how much you know about ancient Rome's famous leaders with our Roman emperor quiz!

The Amazon is the largest rainforest in the world, spanning more than 2 million square miles (5.2 million square kilometers) — an area 12 times the size of California. It influences global water cycles, stores years of global carbon emissions, supports 47 million people, and is home to the greatest concentration of biodiversity on Earth.

But the Amazon rainforest is also disappearing, with 17% of it already cut down or destroyed and largely replaced with agriculture. Other grave threats, such as oil drilling and illegal mining, continue to whittle it down. The next century may have outsize importance, as the forest could reach a "tipping point."

So what will the Amazon rainforest look like in 100 years?

The answer depends on a number of compounding threats, Bernardo Flores, a researcher with the EqualSea Lab at the University of Santiago de Compostela in Spain, told Live Science.

Encroaching farmland and organized crime are a couple of the problems chipping away at the Amazon. But those work in tandem with what he considers the three main threats: climate change, which can lead to extreme weather events, "like wetter wet seasons and drier dry seasons," deforestation and fire.

As the Amazon loses more of its forest, it triggers a feedback loop. "You have less rainfall; then you have less forest, [then] less rainfall, less forest," Flores explained. "That ultimately leads to "a global scale feedback involving the Amazon: More forest loss [leads to] more global warming. More global warming, more forest loss."

As forests get drier, it becomes easier for wildfires to burn more areas. Roads also degrade the forest, and "wherever you have roads, you have people doing illegal activities, illegal logging … then this leads to [more] forest fires," Flores said.

The "arc of deforestation" — a roughly 310,000-square-mile (800,000 square km) border along the Amazon considered the largest deforestation frontier in the world — offers a preview of what much of the Amazon could ultimately look like, according to Flores. The forests that remain there have higher tree mortality and more canopy gaps, and they are often "covered with lianas," or woody vines, that become an ecological problem, he said. Lianas compete with trees for light and nutrients in the soil, and significantly reduce not only a tree's chance of survival but also the overall diversity of trees in a forest. "When the whole forest is covered in lianas, you don't see the forest anymore," he added.

Invasive grasses introduced by cattle farmers will likely proliferate in the decades ahead, but "only a few parts" of the Amazon could become "a savanna, because a savanna is a native, biodiverse ecosystem," he said. Invasive grasses "exclude native species, reduce biodiversity" and would not allow native savanna grasses to replace the forest, Flores said. Instead, one possibility is a "degraded open-canopy ecosystem," where native, naturally fire-tolerant trees, combined with invasive grasses, vines and ferns, proliferate, Flores told Live Science.

Wildlife would quickly be affected as well. Aquatic species are especially vulnerable, Flores said. "When you start having these droughts that will simply last for one, two, three years," wetlands will dry out and become flammable, he explained. That could lead to "very quick extinctions in those areas."

The destruction of the Amazon rainforest would be disastrous for the Indigenous people living there, Christian Poirier, program director of Amazon Watch, an environmental and Indigenous rights advocacy group, told Live Science. "Imagine having your backyard bulldozed and your water source poisoned," he said. "You probably need to move from where you live, and that's exactly what's happening in the Amazon."

A devastated Amazon would also lead to "a more chaotic global climate system," Flores said. There could be less rainfall across parts of South America, and global warming will worsen. Earth could eventually reach a tipping point where ice sheets melt, ocean currents malfunction and the collapse of the Amazon accelerate warming all at once, pushing the planet to "cross the tipping point and transition to a much warmer climate," he said, leading to potentially irreversible consequences.

Unlike other major climate risks, such as the potential of the Greenland Ice Sheet melting and contributing to sea level rise, deforestation can in theory be reversed more easily by reforestation, said Arie Staal, an assistant professor of ecosystem resilience at Utrecht University in the Netherlands.

"That gives us a knob to turn that we don't have for other possible tipping points on Earth," he told Live Science. "It is clear that we really need to stop deforestation in the Amazon. And there's hope."

Editor's note: This article was updated at 6:23 p.m. EDT on June 22 to fix the conversion of roughly 310,000 square miles to 800,000 square kilometers.

Rainforest quiz: Can you sort the largest rainforests on Earth?

It's after dark, and the lights go out; a thunderstorm has knocked out your power. At first, it's pitch-black. But slowly, after a while, the light from the moon makes it possible to see your surroundings. The light didn't change; your eyes did.

But why does it take our eyes so long to fully adjust to the dark?

It has to do with the types of cells in the eye and how they evolved, experts told Live Science.

Our eyes use two kinds of cells to sense light. These cells, known as photoreceptors, are called rods and cones, based on their shapes, Alapakkam Sampath, a retinal neuroscientist at UCLA, told Live Science.

Cones are responsible for color vision. Humans possess three kinds of cone cells, which detect red, green or blue light. The colors people can see are a mix of red, green and blue, Anand Swaroop, chief of the Neurobiology Neurodegeneration & Repair Laboratory at the National Eye Institute, told Live Science.

Rods cannot discriminate colors, but they are far more sensitive to light. Each is capable of detecting a single photon, or particle of light. "Rods are what allow you to see in dim light," Swaroop said.

The extraordinary sensitivity of rods has a price, though: Once each rod detects a photon, it can take up to an hour for it to regenerate its ability to see light, Johan Pahlberg, chief of the photoreceptor physiology group at the National Eye Institute, told Live Science.

The compound that helps rods sense light is called rhodopsin, which is derived from vitamin A, Sampath said. Once each rhodopsin molecule absorbs light, it gets "bleached," meaning it can no longer detect light.

Specifically, rhodopsin is made of two compounds: opsin and retinal, Pahlberg said. When rhodopsin absorbs light, the retinal changes from bent to straight and detaches from the opsin. This retinal eventually makes its way into a part of the eye known as the retinal pigment epithelium, where it can get repaired and resume a bent shape. This retinal can then make its way back to an opsin and reattach to create a functional rhodopsin.

If all of the rods in a human eye are bleached, it may take 45 minutes to an hour for all of them to regenerate, Pahlberg said. However, some rods may regenerate within 10 to 15 minutes to grant some level of vision in dim light, he added.

Rods and cones line the retina, the back part of the eyeball. Rods outnumber cones in most mammal retinas. "In each human eye, there are roughly 6 million cones and 100 million rods," Swaroop said. Sampath noted that "since it takes a lot of time for rods to regenerate, your eyes have a huge number of them to compensate."

Cones are found mostly in the center of the retina, where the eye's lens focuses most of the light it receives, while rods dominate the rest of the retina, Sampath explained. This reflects how during the day, we rely mostly on cone cells for our vision. "Rods are mostly not functional at all during that time," he said.

The human eye does have a quicker way to adjust somewhat to the dark. It can enlarge the pupil, the dark hole at the front of the eye, to let in more light, explained Mark Fairchild, professor of color science at the Rochester Institute of Technology in New York, in an article in The Conversation. However, the vast majority of the eye's adaptation to darkness is due to its rods, and the long amount of time it takes for rods to regenerate explains why it takes so long for eyes to fully adapt to the dark.

Before artificial lighting, humans would typically not experience quick shifts from light to dark. Instead, the ability for human eyes to adapt to the dark would have relied on the setting of the sun, which might take about the same time as rod cells take to regenerate. "There was no evolutionary pressure for it to be faster," Sampath said.

Rods are the most vulnerable cells in the retina to disease and dysfunction, Pahlberg said. This is why older adults often have trouble driving at night. "My colleagues are developing a diagnostic test as a normal part of an eye exam to measure the human adaptation to the dark to see how it changes with age," Sampath noted.

If you've ever washed uncooked rice, you've seen the pale, milky-white water that rinses away. Billions of people across the world eat rice every day. So should everybody who cooks rice wash it beforehand?

This question has been the subject of a number of scientific studies, which have looked at how washing rice affects its texture, nutrition and how well it removes unwanted substances, such as dust, arsenic and microplastics.

Rice is traditionally grown in shallowly flooded fields, as it requires constant irrigation to grow. When the grains are ready, they are harvested and milled to remove the inedible hulls, yielding brown rice, which can be further milled to remove the bran layer, producing white rice.

The act of milling can damage rice kernels, leaving a layer of starch (the substance that makes up much of rice, potatoes and wheat) on the outside. When rice is rinsed, some of this starch is washed away.

In a 2017 study, scientists had speculated that washing rice might change the texture of cooked rice by rinsing off starch that would make grains stick together.

However, subsequent research found that "washing rice makes no difference to the stickiness of the cooked rice," Evangeline Mantzioris, an accredited practicing dietitian at Adelaide University in Australia, told Live Science.

Specifically, a 2019 study showed that "the stickiness of rice is not from the surface starch, amylose, but from another starch within the grain called amylopectin," Mantzioris said. "This is what leaches out during cooking and impacts the stickiness."

The 2019 study found the amount of amylopectin that leaches out of rice during cooking does not depend on whether it is washed. "It is the variety of rice that is important," Mantzioris said. In tests with 10-gram (0.4 ounces) samples of three kinds of rice that were all washed the same amount of time with increasing amounts of water and cooked for 30 minutes, the scientists found that "if you want sticky rice for your dish, you are best using glutinous rice. Medium-grain and jasmine rice are less sticky."

Traditionally, rice was washed for health and safety reasons, "to rinse off dust, insects, little stones and small parts of the husk," Mantzioris said.

However, nowadays, "rice sold in supermarkets and reputable retailers is generally produced under strict quality standards," Bo Wang, a food scientist at Adelaide University, told Live Science. "It is typically cleaned [with machines, such as sifters and blowers], dried, dehusked, milled, graded and packaged before reaching consumers. The drying step particularly reduces moisture content, helping maintain quality and limiting microbial growth during storage. As a result, rice is already a relatively safe product, and washing is generally not required to make it safe for consumption."

Still, "one additional consideration is that some rice may contain naturally occurring inorganic arsenic absorbed from soil and water," Permal Deo, a food scientist with a background in molecular biology at Adelaide University, told Live Science. "Rinsing may help remove some arsenic on the grain surface."

Rinsing rice can also rinse off microplastics, Mantzioris said. A 2021 study found that washing rice before cooking it reduced the amount of plastic contaminating it by 20 to 40%. (It remains uncertain what effect microplastics have on our health, although evidence is growing that microplastics could be harmful.)

Mantzioris noted that washing rice does reduce levels of some important nutrients that naturally dissolve in water, such as copper, iron, zinc and vanadium. However, rice at best offers only a small percentage of one's daily intake of these nutrients. Washing rice, therefore, "is unlikely to affect you nutritionally," she said.

"Long story short, for most consumers, a gentle rinse once or twice before cooking is usually sufficient," Wang said.

This article is for informational purposes only and is not meant to offer medical advice.

Do you know where pumpkins and blueberries come from? Find out with our fruits and vegetables quiz!

As water molecules move around the planet through the water cycle, they take on many forms, moving from solid to liquid to gas and back again. They can make up snowpacks melting in the spring, a river rushing to the ocean, clouds carried on sea breezes, and even pee flushed down the toilet.

But with this complex cycle of evaporation, condensation and precipitation continuing over and over, has the water coming out of your faucet been inside a dinosaur or a mammoth at some point? And does that mean all the water on Earth has been peed before?

The answer depends on how you approach the question.

For Neil Donahue, director of the Steinbrenner Institute for Environmental Education and Research and a professor of chemistry and chemical engineering at Carnegie Mellon University, the answer is likely yes, based on a back-of-the-envelope calculation. But this math is based on a few assumptions, Donahue told Live Science over email.

The first is that the average person weighs about 110 pounds (50 kilograms) and pees about 0.26 gallons (1 liter) per day, which is roughly equivalent to 2.2 pounds (1 kg). Then, Donahue made a "WILD assumption" that all animals pee roughly the same proportion — 1% of their body weight — which he said is "probably wrong" but helpful for a ballpark estimate.

Next, if the total mass of all chordates (the taxonomic group that includes mammals, birds, fish,) is approximately 2.2 billion tons of water and carbon (2 billion metric tons), this equates to around 0.02 gigatonnes — or 8,000 Olympic-sized swimming pools — of pee every day, Donahue said. For the sake of the calculation, he assumed that the weight of chordates has remained constant over time.

On the water side of the equation, the U.S. Geological Survey estimates that Earth has about 1.4 billion gigatons of water, including that in oceans, ice caps, glaciers, lakes, rivers, groundwater and vapor.

If you divide the weight of the world's water by 0.02 gigatonnes of pee each day, it would take around 70 billion days, or about 191 million years, "to pee out the whole ocean," Donahue said. Given that chordates emerged at least 500 million years ago, this is "plenty of time" for all water to become pee at one time or another, he said.

David Kreamer, a professor of hydrology at the University of Nevada, Las Vegas, agreed that when "you go back in history to the dinosaurs and things like that, that's a lot of pee." But he also said calculations that depend on assumptions and generalizations have a large margin of error.

Instead, when it comes to whether every last drop of water on Earth has been peed at one time or another, Kreamer said the answer is no.

For one, water does not move through the water cycle at a constant speed, he noted; water can be trapped as glacial ice for hundreds of thousands of years, and "there is some deep groundwater that's been underground for tens of thousands of years."

What's more, some water hasn't ever been in the water cycle, he said. This water, known as juvenile water, is trapped deep below the planet's surface and "hasn't really emerged from the depths of the Earth ever in Earth's history," Kreamer told Live Science.

Over time, juvenile water can be brought to the surface through volcanic activity in the form of steam or lava. Trapped deep in Earth's crust, "it's released when magma dissolves it, and when a volcano blows up into the atmosphere," Kreamer explained, noting that during eruptions, "there's a certain amount of water or moisture that's released" alongside ash and rock. This water would then enter the water cycle and could eventually find its way into and out of an animal.

Ultimately, at any given time, at sites of volcanic activity around the world, there are streams of new water reaching the surface for the first time. That water "hasn't been peed," Kreamer said ‪—‬ at least, not yet.

Editor's note: This article was updated at 1:41 p.m. EDT on June 15 to fix a math error: 1% of 2.2 billion tons is around 0.02 gigatonnes (8,000 Olympic-sized swimming pools) of pee, not 0.2 gigatonnes (80,000 Olympic-sized swimming pools) as was previously stated. This means it would take approximately 191 million years to "pee out the whole ocean," not 19 million as was previously stated.

As a result, researcher Neil Donahue changed his answer from "emphatically yes" to "likely yes." Despite this error, Donahue says that would still have been enough time if you go back to when chordates first emerged. Previously, he had gone back only to the end of the dinosaur age and said that mammals alone could have done it.

Caves pockmark the world, extending deep into the bowels of Earth. Many caves are record holders, including one in Kentucky that's the longest known cave system, another in Vietnam that's the largest cave, and one in Mexico that holds the title for the longest underwater cave. But caves are known for being dark abysses, so which is the deepest cave on Earth? And which strange animals live at their deepest depths?

There's no clear winner, Live Science found. Instead, there are two caves that alternate as the record holders for the deepest cave on our planet: Veryovkina and Krubera-Voronya (also spelled Voronja), limestone caves in Abkhazia, an autonomous republic in the Republic of Georgia.

Both of these caves are more than 6,560 feet (2,000 meters, or 1.2 miles) deep, and they sit on the same remote mountain range, known as the Gagra Range. Explorers and scientists frequently update each cave's measurements.

According to a list of the world's deepest caves maintained by geologist Paul Burger, a regional hydrologist in Alaska for the National Park Service, Veryovkina is currently in the lead, with a depth of 7,257 feet (2,212 m, or 1.37 miles). Krubera is close behind, at 7,215 feet (2,199 m, or 1.37 miles).

"Both caves are frequently reported with vertical errors of plus/minus tens of feet so first and second place can vary depending on how those data are interpreted," Burger, who is also the author of the book "Cave Exploring: The Definitive Guide to Caving Technique, Safety, Gear, and Trip Leadership" (Myotis Publishing, 2022), told Live Science in an email.

The two caves are located in the Arabika Massif, a glacially eroded karst in the Caucasus. Karst is a type of landscape formed by soluble rocks like marble, gypsum and, in this case, limestone from the Late Jurassic and Early Cretaceous periods, or from about 163.5 million to 100.5 million years ago. Over time, the limestone lays down in layers, and then it gets compressed by tectonic forces and turns into vertical walls of rock, Hazel Barton, a microbiologist and geologist at the University of Alabama, told Live Science.

"If you can imagine all those layers, you've got a BLT sandwich," she said. If you turn that BLT vertically, "when the water goes in, it's always going to look for the easiest way down." The water travels through cracks, eventually hitting the water table. "It's basically a massive bathtub down there," Barton said. Once the water hits the water table, it becomes groundwater and circulates in subterranean streams and rivers.

The reason this region has so many deep caves has to do with mountain building, Barton said "The limestone beds have been tilted near vertically, so the easiest route for the water is just straight down. You also have a large plateau that can collect the water, which helps push it one way or another."

Life in the caves

These caves aren't cozy. They're extremely dark, wet and cold environments. Temperatures inside caves in this region average 35 to 37 degrees Fahrenheit (2 to 3 degrees Celsius) throughout the year, said Ana Sofia Reboleira, a subterranean ecologist at the University of Lisbon in Portugal.

These conditions are harsh, and any organisms living there must adapt in order to survive. The deeper you go into caves, the more limited the nutrients become there. Animals need to slow their metabolism so they can go long stretches without eating, which allows them to live longer, Reboleira said. Their features are also adapted to life underground; most animals living at such depths lack pigment and eyes. They also have long appendages, hair and antennae to help them move in the darkness and sense air vibrations inside the cave.

One of these deep-dwelling species is a type of wingless insect known as a springtail, which Reboleira and her colleagues discovered on a 2010 expedition to Krubera at about 6,500 feet (1,980 m, or 1.2 miles) below the surface. The hexapod, named Plutomurus ortobalaganensis, feeds on fungi and decomposing organic matter. Sixteen years later, it still holds the record for the deepest known land animal.

Subterranean microbes have also evolved mechanisms to survive deep underground. At extreme depths, these microbes rely on chemolithoautotrophy, a metabolic process that enables them to gain energy by oxidizing the bedrock and turning it into food, Barton explained.

By understanding these subterranean environments, scientists can learn more about what's happening underneath our feet. "Caves are windows to the vast dimension of the underworld," Reboleira said.

Caves are ultimately part of a system that is fundamental for all ecosystems on our planet. They provide water purification by filtering water their their many layers, play an important role in nutrient cycling, and sustain the carbon cycle by storing carbon in stone and hosting organisms that turn carbon dioxide into organic matter.

"Subterranean ecosystems are absolutely vital and strategic for ensuring life on Earth," Reboleira said.

How much do you know about our blue planet? Test your terran knowledge with our Earth quiz!

Of the four fundamental forces of nature, gravity is the one we experience most directly ‪—‬ it's what keeps our feet on the ground and the sun in the sky. Yet we still can't pin down its exact strength. Since the 1980s, scientists have made more than a dozen measurements to calculate the precise value of gravity, and many of those numbers contradict one another.

So why is it so hard to figure out how strong gravity is?

One problem is that gravity is weak. Gravity feels strong because we constantly feel the pull of Earth. But the force of gravity between any two objects in everyday life — or any two objects that can fit in an experimental lab — is extraordinarily weak.

"It's weak, and you have to measure this against the background of the Earth's gravitational field," Stephan Schlamminger, a physicist at the National Institute of Standards and Technology, told Live Science. "If we measure gravity, we have to use everyday objects, because these are the only ones where we know the mass. What you have to do in the lab is basically use two very controlled masses, bring them close together, and measure the force between them."

In an April 2026 study, Schlamminger and colleagues replicated a precision experiment to determine the strength of gravity and calculated a value different from the previous result. They used 13 tons (12 metric tons) of mercury to run their experiment, but even then, "the change in the gravitational field was only a millionth of the change that we have here from local gravity," he said.

The team’s measured value was 6.67387x10^-11 m³kg^-1s^-2, which was 0.0235% lower than the previous result — a small difference in everyday terms, but significant in the field of metrology.

Christian Rothleitner, a physicist at the German National Metrology Institute, co-authored a comprehensive review of all gravity measurements with Schlamminger in 2017 but was not involved in the new study.

"This small force has to be determined to six or more decimal places," Rothleitner told Live Science in an email. "This is equivalent to trying to measure the weight of 7 human cells."

Physics, engineering and psychology

One explanation for the discrepancy in values could be that all of the measurements are so imprecise that the true value lies somewhere within them. But each experiment reports a small margin of error, and those ranges don't overlap.

Schlamminger thinks there are three possible reasons for this.

"I have it as a handy-dandy acronym: It's PEP: P stands for physics, E stands for engineering, and the second P stands for psychology," Schlamminger said. "It's sorted by excitement."

The least probable explanation, he said, is the physics one: Maybe there's some element of physics that scientists don't yet understand. Just as general relativity extended scientists' understanding of gravity, there may be another realm of physics yet to be discovered.

"I think it's a remote possibility, but we should not exclude it," Schlamminger said.

Then, there's the engineering explanation: Every experiment uses a slightly different setup, resulting in different values. Some use a torsion balance, a device that senses tiny forces by measuring the twisting of a small fiber. Others use pendulums or free-falling objects. Each approach has its own potential sources of error, and those mistakes are difficult to untangle from the gravitational signal.

"I personally do not believe that the reason lies in the physics, but in the measurement technology," Rothleitner said.

Human error is another part of the engineering explanation. "Such an experiment requires expert knowledge in many areas of physics and measurement technology," Rothleitner said. "You cannot be an expert in all those fields. This kind of measurement is on the cutting edge of measurement science."

The most likely possibility, Schlamminger said, relates to psychology.

"There is a driver for these people who measure these numbers to give really, really small uncertainties" — that is, margins of error — "because it makes them famous," Schlamminger said. "Because the pressure is there, the uncertainties may be a little bit too small, and that's why they don't agree with each other."

In the end, though, a precise measurement of gravity may not matter. We know the product of G times Earth's mass, and that's enough for practical applications like launching rockets into space. That may be all we need for now.

"The value of Newton's gravitational constant is rather of academic interest," Rothleitner said. "If it were different, nations would have spent much more effort in determining it better."

Schlamminger still finds it exciting, though. "We live in a society where we think everything is discovered," he said. "But if you look, there's still terra incognita. There are still problems, and the problems may be small, but they're still problems we can solve and contribute to and find mesmerizing and intriguing. And this is one of those problems."

Whether using family records, DNA tests or genealogy websites, many people can trace their family histories back generations. The world-record holder for the longest family tree is Chinese philosopher Confucius (551 to 479 B.C.), whose family tree extends more than 80 generations from his ancestors in the eighth century B.C. to his living descendants. That's almost 3,000 years.

But our species has been around for 300,000 years, based on scientific dates of the oldest known fossils. So how many generations do we go back as a species?

To find out, you need to know how long modern humans (Homo sapiens) have existed and the length of a generation, according to Matthew Hahn, a population geneticist at Indiana University Bloomington. The number of human generations that have lived would be equal to the time since H. sapiens emerged as a species divided by the length of a generation, also called the generation interval.

The generation interval is typically defined as the average age at which humans have children, Hahn said. It tends to be longer for men than for women because men can have children later in life, Hahn explained.

There are many estimates of our species' generation interval, each of which produces a different answer to the question of how many human generations have come before us.

For example, a 2003 study of Icelanders published in the American Journal of Human Genetics based generation interval estimates on the country's extensive records from churches and other sources. Using these records, researchers at the company deCODE Genetics created a genealogical database of the country's entire population. They found that the average generation interval in Iceland over the past 300 years was 30.3 years.

A 2005 study used data about the average age at which European women had children between 1960 and 2000 and estimated the generation intervals for men to arrive at an average generation interval of 29.1 years.

But those are estimates of the generation interval in the recent past. A study led by Hahn and published in the journal Science Advances in 2023 estimated the generation interval over the past 250,000 years. Hahn's study put together data from two others. A 2017 study of Icelanders, also led by deCODE Genetics and published in the journal Nature, found that as parents age, the blend of mutations that arise in their children changes. Using this data, Hahn and colleagues built a model of the mix of new mutations that you would expect to find in a group of people according to the generation interval at the time.

"If you know the types of mutations that individuals leave to their children according to their age, if you have a collection of mutations, you can try to estimate how old the mixture of individuals was," Hahn said.

A 2020 study in the journal PLOS Biology estimated when millions of mutations found in humans today first arose. Hahn and colleagues grouped the mutations from the 2020 paper according to when they arose and then determined the blend of new mutations that popped up during each time period. With that information, they could estimate the generation interval for each time span. While the generation interval varied over the course of an estimated 250,000 years, it was an estimated 26.9 years, on average. Using that generation interval, there would have been an estimated 11,152 generations of humans over 300,000 years, Hahn said.

Moisès Coll Macià, an evolutionary biologist and population geneticist who works as a postdoctoral researcher at the Institute of Evolutionary Biology in Barcelona, Spain, told Live Science that while a generation interval of 26.9 years is "not unimaginable," he prefers to give a range of possible generation intervals.

In Coll Macià's view, the lower bound would be the generation interval for one of our closest living relatives, chimpanzees (Pan troglodytes). Because humans and chimps share a common ancestor that lived during the Miocene epoch (23 million to 5 million years ago), you would expect that past human generations would have had a generation interval somewhere between that of contemporary humans and that of contemporary chimps, Coll Macià said. Chimpanzees have an estimated generation time of about 24.6 years, according to a 2012 paper in the journal PNAS.

As for the upper bound in modern humans, Coll Macià suggested 26 to 30 years. That's based on a 2016 PNAS study that analyzed fragments of Neanderthal DNA found in ancient and contemporary human genomes to estimate the human generation interval across the past 45,000 years.

Based on Coll Macià's upper-bound generation interval of about 30 years, there have been at least 10,000 generations of humans. Based on the lower-bound generation interval of 24.6 years, there have been at most 12,195 generations. These numbers attest that the human family tree is pretty tall, however you look at it.

See how much you know about early humans with our human evolution quiz!

Most long-distance travelers have felt the discombobulation of changing time zones: being excessively tired during the daytime but struggling to sleep at night.

Jet lag often emerges when people hop several time zones. The majority struggle more when flying east because our body clock is typically just over 24 hours. "It's a little bit harder to shift earlier," Helen Burgess, a professor at the University of Michigan's Sleep and Circadian Research Laboratory, told Live Science.

So what's going on in our bodies when we have jet lag, and is there a way to prevent it?

What is jetlag?

Jet lag is a mismatch between our internal body clock and our time zone.

The term "jet lag" was coined in a 1966 Los Angeles Times article, in which journalist Horace Sutton described the symptoms experienced by long-haul jet-setters as "not unakin to a hangover."

"He said, at the time, that jet lag derives from the simple fact that jets travel so fast they leave your body rhythms behind," Dr. Charles Czeisler, a professor of sleep medicine at Harvard Medical School, told Live Science. Before flights, people traveled overland or via ship so slowly that their circadian clock didn't get out of alignment.

Jet lag is the body's normal response to skipping time zones. "If the circadian system is working correctly, you should have a problem with jet lag," Czeisler said.

But some populations — such as older people, frequent fliers or those with mood or sleep disorders — are more likely to feel symptoms.

Jet lag might seem trivial, but even a small dose can have serious health implications. When the clocks move forward — essentially causing an hour's jet lag — there's an increase in fatal car accidents, Czeisler said. Getting behind the wheel straight after a long-haul flight is dangerous because a lack of sleep is a known cause of car crashes. In fact, road accidents are the leading cause of non-natural death among U.S. citizens traveling abroad.

And shifting circadian rhythms can even "flip people into mania or depression," Czeisler said. According to one study, 186 people over two years were admitted to a psychiatric hospital from Heathrow Airport with jet-lag-related mental health conditions, such as depression and hypomania (a shorter, milder form of mania), while another study has shown that chronic jet lag could increase the risk of neurological disorders.

Minimizing jet lag

The best way to adjust to your new time zone is through light exposure. "Light is the strongest signal that shifts the timing of the clock," Burgess said.

But the timing has to be right: exposure to light at the wrong time can push your clock in the wrong direction, making jet lag last longer.

Around two or three hours before your usual wake-up time, your body clock switches from interpreting light as evening light, making you sleepy, to morning light — which wakes you up.

"Light before the crossover point will shift you later, and light after the crossover point will shift you earlier," Burgess said. This is why napping isn't always the right solution.

If you usually wake up at 6 a.m., your crossover point will be around 3 a.m. This means that if you travel to a time zone six hours ahead and land at 7 a.m., this would be around 1 a.m. for your internal clock, which is still on your original time zone. So, your body interprets all the morning light you take in while landing and going through security as evening light and it wakes you up again, pushing your clock later.

Knowing when to expose yourself to light requires planning. To get light when you need it, go outside, use light boxes, or put on wearable light therapy devices. To avoid light when it will push you in the wrong direction, nap, stay inside, or wear sunglasses or glasses that block blue light within a specific wavelength range (460 to 480 nanometers).

To minimize the effects of jet lag, stay hydrated, avoid alcohol, be mindful of not drinking too much caffeine — "maybe just one cup of coffee" at strategic times, Burgess said, adding that caffeine has a long half life so will stay in your body and make it harder to sleep — and adjust to local mealtimes as soon as possible. Some people may also speak to medical professionals about whether melatonin is suitable for them.

You can prevent jet lag by shifting your body clock to the new time zone before you arrive. "On average, we can only shift the clock earlier by about an hour and a half a day," Burgess said.

For instance, to adjust to a time zone 10 hours ahead, you would need to start shifting your light exposure timings a week before the flight. For example, if you usually sleep between 11 p.m. and 6 a.m., you'd be aiming to shift toward going to bed at 9 a.m. and waking up at 4 p.m — something not feasible for most people.

It works, Burgess said, but when she tried it, she felt disconnected from the world around her because everyone was asleep during her waking hours. "I just felt really, really isolated," she said.

Although it's impractical for most people, you wouldn't experience the symptoms of jetlag because of how gradually you'd be moving your sleep schedule toward the new time zone. "People do not experience jetlag symptoms because everything is being moved slowly together," Burgess said, not "yanked apart" in one flight.

If you don't want to become nocturnal before you leave, nudge yourself in the right direction by shifting your schedule earlier by around an hour each day for just a few days. In essence, if fully switching to the new time zone before you depart is too challenging, nudging closer to it before arrival will still be beneficial. "It can make a huge difference," especially for those flying east, Burgess said. "Take the five or 10 minutes to plan it out, and that's going to really help."

This article is for informational purposes only and is not meant to offer medical advice.

Around 66 million years ago, a gigantic asteroid smashed into Earth and wreaked chaos globally.

Superheated rock from the impact spewed into the air, creating a mushroom cloud that heated Earth's upper atmosphere to a scorching 439 degrees Fahrenheit (226 degrees Celsius). Mile-high tsunami waves rushed through the Gulf of Mexico and disturbed ocean basins half a world away. Fires raged, burning animals and plants to a crisp. Shock waves propagated, blasting everything in their path. And particles from the collision, including sulfur, shot upward, blocking the sun and falling down as acid rain.

In all, 75% of Earth's species went extinct, including the nonavian dinosaurs. So how did some animals ‪—‬ including species of birds, turtles and mammals ‪—‬ survive the catastrophic asteroid collision and its aftermath?

Size, it turns out, played a crucial role. Earth's largest apex predators and herbivores — dinosaurs like Tyrannosaurus rex and Triceratops, and marine reptiles like plesiosaurs and the behemoth Mosasaurus — were doomed from the moment of impact, Kenneth Lacovara, founding executive director of the Edelman Fossil Park and Museum of Rowan University in New Jersey, told Live Science. That's because their enormity meant they were more likely to be harmed during the initial blast, were unable to hide in a safe place during the apocalyptic aftermath, and needed massive amounts of sustenance to survive in a time when food was scarce.

It's not too surprising that size would be linked to survival, Lacovara said. On land, "it's pretty clear in terms of correlation that you have to be small, and you have to be a burrower to get through this event," said Lacovara, who is also a professor of paleontology and geology at Rowan University. Small animals, such as some lizards and mammals, which were no larger than badgers at that time, were more likely to find shelter from the asteroid's immediate and long-term aftereffects. Other small animals that required less food, such as some turtles and fish, were sheltered in the water.

The avian group that led to modern birds survived likely because they were small and had powerful wings and chest muscles that allowed them to fly well and escape dire situations or find new opportunities. Their chicks also grew quickly, meaning they could soon fend for themselves and not overtax their parents.

This led to an overall shift in the average size of Earth's animals. On land, most of the largest surviving animals were about the size of house cats; in the water, the largest survivors were about the size of a "run-of-the-mill shark," Lacovara said.

What the survivors ate

Diet was likely another important factor, said Roger Benson, a curator of dinosaur paleobiology at the American Museum of Natural History in New York City. The plant eaters and those that ate them were especially hard-hit, as the sun was obstructed for up to a decade. Even with size on their side, some smaller creatures, such as certain lizard and turtle species, went extinct because their diets were too reliant on photosynthesizing plants, Benson said.

Aquatic ecosystems were a bit more shielded from the asteroid's initial impact, especially in deeper oceans and freshwater ecosystems. But as photosynthesizing plankton died off due to a lack of sunlight, food systems collapsed and large marine animals starved. Those that consumed dead, organic detritus had a better chance of survival. Some of those resilient marine creatures included the sea sponges, sharks of the Carcharias genus, and mollusks, including the lineage that led to today's chambered nautilus (Nautilus pompilius).

Seed eaters, including birds, and foraging insectivores, such as the tree-dwelling primate Purgatorius janisae, had a better shot at surviving because their food sources of seeds and insects weren't destroyed by extreme temperature changes and a lack of sunlight.

A common theory is that species with more generalist diets had a better chance of surviving the drastic environmental change. For example, Purgatorius coracis, a small mammal, survived the mass extinction thanks to their wide-ranging diet of insects, fruits and seeds. (This trend holds true for modern animals facing climate change: Those with generalist diets, such as crows and raccoons, are expected to do well because they have a wide range of foods to fall back on in case one food source disappears.)

Some species got lucky if their prey were also survivors. A few aquatic turtle species, like Hutchemys rememdium, had an adaptive feeding habit of eating shelled creatures which were living off detritus in aquatic ecosystems. A 2026 study found this adaptation, called durophagy, was linked to comparatively high survival rates during this mass extinction.

Certain behaviors, such as a higher capacity for reproduction or behavioral flexibility in a changing environment, may have enabled survival Benson said.

Mysterious exceptions

Despite current evidence, there are still unknowns surrounding mass extinction survivorship. Recent research indicates we don't know why certain adaptations helped some species but not others.

For example, many present-day species of bivalves feed on microscopic aquatic organisms that rely on the sun to survive. However, among the aquatic bivalves that survived the asteroid and its impacts, reliance on the sun didn't strongly determine survivorship. In another case, night lizards that survived near the impact site are known to have small litters, which goes against the hypothesis that fecundity is advantageous following extinctions. However, these night lizards' slow metabolisms were likely helpful for their survival, and they persist today, the study noted.

And the discovery that a large, terrestrial croc (Tewkensuchus salamanquensis) survived the asteroid strike in what is now Argentina raises questions about how this 660-pound (300 kilogram) species survived. This also raises questions whether the asteroid's impacts weren't as strong in the Southern Hemisphere as they were in the Northern Hemisphere. For example, the wide diversity of plant fossils uncovered in present-day Argentina suggests plants were able to regrow faster at southernmost latitudes.

"We also don't know why mammals emerged from the extinction as the dominant macrofauna," Lacovara said. One theory is that mammals are more resistant to fungal infections than reptiles are, giving mammals a better shot at surviving the "mini age of fungus" that followed the mass extinction.

After all, how evolution proceeded following the mass extinction is a part of humanity's history. The animals that survived ultimately ushered in the age of mammals, including humans, that continues to this day.

What do you know about crocs? Test your knowledge with our crocodile quiz!

When Earth first formed around 4.5 billion years ago, it was a ball of molten rock. Over time, heavier elements, like iron and nickel, sank to the planet's center, forming the Earth's early core.

Today, Earth's core remains an incredibly hot and dense sphere deep inside our planet. It consists of a liquid outer core, which starts at around 1,800 miles (2,900 kilometers) below Earth's surface and extends for 1,400 miles (2,200 kilometers). There is also a solid inner core, which begins at around 3,200 miles (5,150 km) below ground, with a radius of roughly 758 miles (1,220 km).

But just how hot is Earth's core? And how did scientists figure it out, if they can't go that deep underground?

Thanks to a combination of techniques, scientists have estimated that the temperature of the Earth's core is about as hot as the surface of the sun: It reaches around 9,000 to just over 10,000 degrees Fahrenheit (roughly 5,000 to over 5,500 degrees Celsius). This temperature comes from the boundary between the inner and outer core, which is thought to be the hottest part of the core.

However, this temperature was not measured directly. Instead, it is inferred via experiments and theories that scientists have of the composition of the core.

Earth's center is composed primarily of iron, roughly 85%, alloyed with nickel and other lighter elements; this material is in liquid form in the outer core and solid in the inner core. Scientists deduced these properties from a mix of laboratory measurements of iron alloys at high pressures, analyzing the composition of meteorites, and understanding how seismic waves bend or disappear as they travel through the planet's interior.

Because Earth's outer core is made mostly of liquid iron, temperatures in this region must be higher than iron's melting temperature. At the planet's surface, the melting point of pure iron is 2,800 F (1,538 C). But this number doesn't take into account the "enormous pressures" of the deep interior, Quentin Williams, a mineral physicist at the University of California, Santa Cruz, told Live Science. Increasing pressures boost the melting point of iron and most other substances, which explains why the inner core is very hot, but remains solid due to its high pressure.

To determine the melting temperature of iron at astronomical pressures, scientists have conducted a number of experiments to simulate this environment. Some studies have squeezed a piece of iron between two sharpened diamonds (called diamond anvil cells) to generate high pressures while a laser heated the iron to high temperatures. Others have hit pieces of iron with high-velocity projectiles or shock-creating rays to simulate crushing pressures. The results from those experiments were then plotted and extrapolated to the pressures at the boundary of the inner and outer core, which led to the estimates ranging from around 9,000 to just over 10,000 F.

"To some extent, what we know about the Earth['s] core is all an educated guess," Shichun Huang, a geology professor at Sun Yat-sen University in China, told Live Science. Many mechanisms, such as how the solid inner core crystalizes into a solid, are still a mystery.

Hot from the beginning

All of this heat hints at our planet's unique history. When Earth formed, all sorts of material got pulled together, including the iron that makes up the core. That "gravitational potential was converted to heat," Huang said.

Further, scientists think that some time during that formation, a Mars-size object hit our protoplanet and that this force deposited a lot of heat into the interior. Some scientists think that radioactive elements, like potassium, uranium and thorium, also contribute to the planet’s internal heat, although whether these elements are actually present in the deep Earth is debated, Huang said.

A hot core also contributes to Earth's ability to host life. In contrast to other planets, Earth's interior has held onto much of its original, primordial heat.

"We're not really good at planet cooling," Williams said, meaning that Earth has held onto much heat from its original formation, unlike other rocky planets in our solar system. As a result, our planet has properties like plate tectonics, which moves pieces of the Earth's surface, bringing up nutrients and creating diverse habitats for life to evolve and thrive. The iron core that is part liquid also generates Earth's magnetic field, which protects the planet and life on it from dangerous solar winds.

"If you care about life, you should care about the inside of the Earth," Huang said. A blazing hot core in the center of our planet is what allows all of us to survive where we are today.

What is Earth made of? Find out with our inside Earth quiz!

At the Alcor facility in Arizona, more than 150 disembodied heads reportedly lie in cryogenic chambers, preserved in hopes that future medical advances can bring these brains back to life in new bodies. Given that scientists still cannot revive a cryogenically preserved brain, why do patients bother with cryonics at all? Why couldn't these heads just be stitched onto new bodies in the present day, when they're still fresh? In other words, why isn't a brain transplant possible?

Dr. Max Krucoff, an assistant professor of neurosurgery at the Medical College of Wisconsin, would rather describe such a procedure as a body transplant. Unlike a patient who receives a donor heart or liver, transplanting a brain into a patient's body would make them "a completely new human being," he told Live Science. "Your agency, your identity, is contained within your brain."

Semantics aside, Krucoff said this transplant is currently impossible because surgeons cannot yet forge signaling connections between nerves in the central nervous system, which includes the brain and spinal cord. Transplanted peripheral nerves, or the nerves that branch off throughout the body beyond the brain and spinal cord, can eventually communicate with their new neighbors, since these nerve cells can regenerate. There is less evidence that the adult human central nervous system can generate fresh neurons, and if it does, it's to a limited degree. Neurons can form new connections throughout a person's lifetime, which is how learning is encoded, but scientists do not understand this pathway well enough to exploit it for a transplant.

Even a partial brain transplant, such as a cerebellum swap, is out of the question for now. In the case of the cerebellum, the structure is home to millions of specialized neurons called Purkinje cells, each of which receives signals from thousands of other neurons. "The number of connections is exponential," Krucoff said. "That's way beyond our capacity."

Fusing brain and body at the spinal cord would be the simplest theoretical brain transplant, since connections between spinal nerves are more straightforward than neurons in the brain. The surgeon performing this head transplant could connect the skin, muscle, vessels and bones in the neck, and even align the nerves of the spinal cord, "but to get those cells to communicate, we just haven't figured out how to do that yet," Krucoff said.

Has a brain transplant ever been attempted?

Scientists began attempting head transplants on animals with the advent of new blood vessel suturing techniques in the early 1900s. Most of the dogs and monkeys involved in those experiments survived a few days at most, as researchers struggled to create a working vascular system or to manage host rejection of the transplanted head with immunosuppressive agents.

Starting in 1970, Dr. Robert J. White transplanted the heads of monkeys onto new host bodies. The animals could chew and swallow food after surgery, and postsurgical electroencephalogram readouts suggested that their brains were awake, though none survived more than nine days.

Inspired by this work, Italian surgeon Dr. Sergio Canavero laid out his vision for the first human head transplant in 2013, attracting widespread pushback from colleagues on ethical and scientific grounds. Canavero's 2017 announcement that he had performed the first head transplant on a human cadaver was described at the time as "the continuation of a despicable fraud" by New York University bioethicist Arthur Caplan, who pointed out the likelihood of immune rejection and the challenge of linking a brain with all-new nervous inputs.

Can any tissue be transplanted into the central nervous system?

Although a surgeon can't switch one brain out for another, stem cell or organoid grafts could one day replenish damaged or diseased brain tissue.

Stem cells programmed to develop into neurons may have better odds of integrating into existing circuitry than mature neurons do, said Ruslan Rust, an assistant professor of research physiology and neuroscience at the University of Southern California Keck School of Medicine.

Ideally, these stem cells could be derived from the patient's own tissue, to lower the odds of immune rejection, but standardized donor cell lines reduce the need for quality control on individual tissue samples. In this way, neurons originating from one person's tissue could populate another person's brain.

Stem cell therapies have been tested in clinical trials for Parkinson's disease, stroke, spinal cord injury and epilepsy, but none of these treatments for these conditions are approved by the U.S. Food and Drug Administration for commercial use.

Rust told Live Science that future research will need to address the potential risks of these techniques, such as incompletely differentiated stem cells forming a tumor after transplant, or cells that do become neurons disrupting signaling pathways.

"The billion dollar question is, how do we make [transplanted cells] the cells that we want them to be, and how do we make sure that they integrate into those local circuitries where we want them to go?" he said.

Stem cells are also used to create lab-grown models of nervous tissues, called organoids, that can be transplanted into the brain. A 2023 study reported that human brain organoids could repair injured rat cortex, though therapies based on organoid transplant are still years away. Rust said such procedures could be more invasive than transplanting individual stem cells, and would need to account for vascular supply to the new tissue.

See how much you know about the most complex organ in the human body with our brain quiz!

It's a plotline that has haunted science fiction for decades: Humans become infected by plant pathogens, causing illness, death or a zombie-like state. But is there any truth to this idea? Can plant germs really cross over to people?

The short, disturbing answer is yes, experts told Live Science, but it's incredibly rare.

Plant pathogens are viruses, bacteria, fungi or other microorganisms that cause disease in plants, usually by infecting cells, reproducing, and spreading within and feeding on plant tissues. This can cause cell death, wilting, peculiar growths and discoloration; interrupt plants' ability to photosynthesize; stunt their growth; and, ultimately, kill them. Fungi cause an estimated 85% of plant diseases overall.

Much of what we know about how the human body functions makes it seem unlikely that these pathogens could invade our bodies, too. "It is extremely rare for a pathogen to jump from a plant to a human or vice versa because our biological architectures are fundamentally different," Dr. Soma Dutta, a physician and medical microbiologist at Apollo Multispeciality Hospitals in Kolkata, India, told Live Science over email.

Plant pathogens have evolved to breach the thick, rigid walls of cellulose or hemicellulose inside plants. By contrast, human cells are protected by lipid membranes, which are lined with receptor proteins that can initiate an immune response. Plant pathogens generally don't have the tools to break through these membranes, Dutta explained.

Besides, for most plant-specific fungi and bacteria, which typically survive in temperatures of up to 77 degrees Fahrenheit (25 degrees Celsius), our much higher bodily average is uncomfortably hot. "The human body is essentially a furnace," Dutta said. "The heat denatures their proteins and prevents them from causing infection."

Deadly pathogens

But there are exceptions. In 2023, Dutta and colleague Ujjwayini Ray, a consultant microbiologist at Apollo Multispeciality Hospitals, Kolkata, published a case report abouta patient who had developed a persistent cough and sore throat, was struggling to swallow, and was not eating, with seemingly no cause. When doctors examined him, they found a pus-filled abscess developing beside his windpipe. An exam revealed spores from Chondrostereum purpureum, a fungus that causes silver leaf disease in plants.

The researchers discovered that the 61-year-old was a plant mycologist, someone who studies the relationship between fungi and plants. "For years, his professional work involved direct, intense contact with decaying wood, mushrooms, and various plant fungi. This provided a clear pathway for inhalation of spores," Dutta explained. She and her colleague hypothesized that the spores had somehow managed to "hide from or resist" phagocytosis, a crucial phase of the human immune response, allowing them to establish a colony and spread. But to Dutta, the fungal infection remains a puzzle, because despite being a cool-weather fungus, it had managed to survive the higher temperatures of the human body.

With a course of antifungal tablets, that patient recovered fully. But that's not always the case; in the rare situations where plant pathogens cross over into humans, they typically affect those with weakened immune systems, which can result in death. Many of these pathogens thrive in warm, moist, stable environments, and they have been detected in catheters and breathing equipment in hospitals, which is how vulnerable patients can become exposed.

For example, the bacterium Pantoea agglomerans causes disease in a range of food crops ‪—‬ like rice, maize and sorghum ‪—‬ and if it gets into the human bloodstream, it can be deadly. Another type of bacteria, Burkholderia, causes rot in onion and rice, and research has documented cases where this germ has infected cystic fibrosis patients, causing pneumonia, blood poisoning and death. An investigation into several strains of B. cepacica, found that the microorganisms could surpass the defensive cilia and mucous lining the airways and get into and spread within the lungs.

Another bacterium, Pseudomonas aeruginosa, causes soft rot in plants like lettuce and potatoes. But it has also been found in hospitals, where it has passed to immunocompromised patients with severe burn wounds, cancer or AIDS, causing infections in their urinary tracts, blood and lungs.

Plant viruses

Plant viruses are generally believed not to pose a risk to humans because of the unique mechanisms they use to infect plants, but scientists have unearthed some compelling clues that we might be vulnerable to the diseases they can trigger. In 2014, Philippe Colson, a professor of medical virology at Aix-Marseille University in France, delved into the strange case of pepper mild mottle virus, a highly contagious plant pathogen that causes pepper plants to shrivel up. Peppers are widely consumed, so Colson and colleagues studied the stool samples of over 400 people and found the RNA of the virus.

Individuals who had the virus in their stools were significantly more likely to have symptoms of disease. "We were able to find a correlation between the presence of this virus in human stools and the symptoms of fever and pruritus [itchy skin]," Colson told Live Science. Although they didn't show a definite causal link ‪—‬ the symptoms may have been triggered by something else in the food ‪—‬ "it made us question whether there could be an interaction with our body, and whether or not it could be pathogenic in some cases," he said.

The study also showed that the affected patients' stools contained antibodies specific to the pepper virus. The virus's ability to withstand the human gut suggests it is highly resilient; in fact, pepper mild mottle virus is so robust that it's been considered for use as an indicator of human fecal pollution in waterways.

Colson is also intrigued by tobacco mosaic virus, which leaves a distinctive pattern on tobacco leaves that causes shriveling, affects their ability to photosynthesize, and stunts their growth. Strikingly, previous research into this virus had "looked at people that had lung cancer, and they found the virus in their biopsies," Colson said. Colson's own research has detected tobacco mosaic virus in several cigarette brands and in the saliva of people who smoked them. This raises tentative questions about whether the virus can travel into human bodies through cigarettes and possibly play a role in smoking-related cancers, Colson said.

For now, research on viral plant pathogen crossover into humans is limited, with the central puzzle being how they might infect humans if they do; plant viruses typically enter plant cells through a hole in the cellulose wall, whereas viruses in humans have to interact with a series of receptors around the cell before they can break in, Colson explained. Yet the important implications of potential plant virus infection in humans makes him "quite confident that this is a field that will expand in the near future."

The research so far shows that it's highly unusual for any plant diseases — fungal, bacterial or viral — to seriously, or fatally, develop in people who don't already have severely compromised immune systems. Crossovers are "extremely rare," Dutta noted, but she believes that our changing planet means plant pathogens deserve continued attention and research.

"The most significant barrier of the human body is temperature, which is being eroded by climate change and global warming," Dutta said. As global temperatures rise, plant pathogens may be finding ways to adapt to these warmer conditions, equipping them to survive in our bodies.

"The final message regarding plant-to-human crossovers is one of evolving vigilance," Dutta said. "While the biological barriers between plant and animal cells remain very strong and not easily breakable, the case of the Chondrostereum purpureum infection serves as a vital reminder that 'rare' does not mean 'impossible.'"

They sleep on our beds, steal our food, and generally rule the house. Between them, cats and dogs make up two-thirds of pet ownership worldwide. But which of the two companion animals are we more closely related to?

The answer depends on how you look at the question.

"From an evolutionary perspective we are equally related to dogs and cats," Mark Springer, a professor emeritus of evolution, ecology and organismal biology at the University of California, Riverside, told Live Science in an email.

Cats, dogs and humans are all mammals. In the mammalian family tree, which maps out how different mammal species are related to one another, both cats and dogs belong to the order Carnivora, while humans are primates. These two groups split from a common ancestor about 90 million to 95 million years ago, Springer said. Meanwhile, cats and dogs split from each other much later, around 55 million years ago.

In terms of common ancestry, "dogs and cats are more closely related to mammals such as pangolins, horses, cows, whales, bats, shrews and moles than they are to humans," Springer said. And "humans are more closely related to colugos [flying lemurs], tree shrews, rabbits, rats, and mice than they are to cats and dogs."

Genetic ties

Another way of deciding which species we are more closely related to is through a genetic lens.

If you measure how much the DNA code has changed over time, humans are about equally related to cats and dogs, William Murphy, a comparative genomicist at Texas A&M University, told Live Science in an email.

However, scientists also compare how DNA strands are organized within chromosomes. Here, a difference emerges.

Murphy explained that the ancestors of modern-day dogs went through extensive chromosome rearrangements over evolutionary time. (Such rearrangements are not unique to dogs; they occur across different animal and plant species, though scientists don't fully understand why some lineages rearrange faster than others.) Cats, on the other hand, retained a genome organization that's closer to ours. "In terms of how genes are arranged within chromosomes, humans and cats are twice as similar to each other as humans are to dogs," he said.

Because the way DNA is organized affects how genes are switched on and off, cats may be a better model than dogs for understanding human gene regulation, Murphy said.

That also makes them useful for studying genetic diseases. For example, polycystic kidney disease occurs in both humans and cats, and treatments developed for cats could help inform therapies for people.

Cats may also provide clues about cancer. A recent study found that cancer-related genes in cats are strikingly similar to those in humans, both in number and variety. One notable example involves a gene called FBXW7, which was mutated in more than half of the feline mammary tumors studied. In humans, mutations in the same gene are linked to worse outcomes in breast cancer

That said, dogs are also used to model and analyze human illness, including Alzheimer’s disease, idiopathic epilepsy, eye disease and heart disease.

Although cats may share more similarities with humans in gene regulation, to date more research has focused on dogs. This may be due in part to the fact that the complete feline genome became available later than the canine genome, as well as historical bias — cats have long been perceived as less cooperative in research settings.

So, which are we more closely related to? From an evolutionary standpoint, it's a tie, but genetically, at least in terms of genome structure, we are closer to cats.

How much of a cat fan are you? Find out by taking our cat quiz!

Over the past two decades, some studies have suggested that human brains are shrinking. But there is also evidence that IQ scores have risen over the past century.

But is it possible for us to get smarter as our brains shrink? Live Science contacted experts to find out.

First, it's important to note that a bigger brain size does not necessarily mean higher intelligence, said Jeremy DeSilva, an anthropology professor at Dartmouth College. Brain size "is only weakly related to measures of intelligence in humans. Albert Einstein's brain, for instance, was quite small, and he was Einstein!" he told Live Science in an email.

In Einstein's case, it appears that extraordinary folding patterns in several of his brain regions may help account for his genius. While there is some debate, studies suggest that there is little or no relationship between our intelligence and brain size in humans.

Have human brains shrunk?

Not all scientists agree on whether human brains have gotten smaller. However, many experts we spoke with said there is evidence for shrinkage over time.

"My research indicates that human brain size declined during the [entire span of the] Holocene by about 10% of its volume or about 150 ml on average," Maciej Henneberg, a professor emeritus of anthropological and comparative anatomy at Adelaide University in Australia, told Live Science in an email. The Holocene is an epoch that followed the last ice age, and spans from 11,700 years ago to the present day. In his research, Henneberg looked at the brain size by analyzing skulls from all over the world. In many cases, he analyzed the skeletons personally.

It's important to remember that Homo sapiens emerged around 300,000 years ago, so a brain shrinkage starting around 11,700 years ago would be a relatively recent development.

Other scientists have reported similar findings. "Our lab thinks that the data currently available show a clear, global trend toward a decrease in brain size in more recent times," said DeSilva, whose research looked at more than 5,000 skulls from people who lived in Europe, Asia, Africa and Australia. Many of the skulls date to the Holocene.

Jeff Stibel, who holds a doctorate in brain science and has published a number of papers on the topic of brain shrinkage, said the "Holocene warming period has coincided with more than a 10% reduction in brain size in modern humans." Stibel took part in DeSilva's research, and then gathered more data by analyzing the brain size of roughly 800 additional skulls from around the world.

However, some scientists disagree. Research by Brian Villmoare, an associate professor of anthropology at the University of Nevada, Las Vegas, and his team found no evidence that human brains are shrinking. "I see no evidence that, once we acquired our modern form, that our brains have changed in any meaningful way," Villmoare said in an email.

Some say more nuance needs to be considered. "Brain size in some human populations decreased over the last 15000 years," John Hawks, an anthropology professor at the University of Wisconsin-Madison, told Live Science in an email. "But there are some complexities."

Hawks noted that datasets of brain size tend to overrepresent men of European ancestry, making it difficult to determine whether there is a global trend among diverse populations.

"Brain size did rebound toward larger sizes during the last 150 years in industrializing countries," Hawks said. "This was likely mostly attributable to nutrition and correlated with body size, but whether body size accounts for the entire effect is not clear from the limited data."

Why are human brains getting smaller?

If our brains have shrunk, then why? Scientists have proposed a few ideas.

One has to do with the introduction of farming.

"During the Holocene, humans gradually introduced food production [such as agriculture and animal husbandry] that allowed them to live in larger communities," Henneberg said. "The brute physical strength required for hunting big game and protecting families against strong predators became less necessary while a smaller body size required less food, so it was favoured by natural selection."

In fact, it's not just our brains; our bodies have also shrunk. "At the end of the Ice Age, male body height was about 1.75 m [5.74 feet], whereas in mid-Holocene agricultural communities it was 1.65 m [5.41 feet]," Henneberg said. "Body mass has declined even more since we see thinner (less robust) human bones." Heights have rebounded recently in some parts of the world, and Hawks said that our brain sizes may be rebounding also.

The warming that occurred after the end of the last ice age also may be a factor. Stibel said two biological principles, known as Bergmann's rule and Allen's rule, state that "bodies and organs tend to become leaner in warmer climates to increase surface area and dissipate heat."

Another possible explanation is that human intelligence itself has changed as we have taken on more specialized jobs and share more information with each other, meaning we don't have to know everything, just a subset of knowledge to keep society going.

DeSilva said his team proposed that a "population increase, the subsequent specialization of people into specific roles in society and the growth of collective intelligence may have contributed to individual brain size decrease as modeled in some eusocial insects like ants and wasps." For instance, "I'm a decent enough anthropologist and anatomist, but you wouldn't want me fixing your car or investing your retirement savings on Wall Street."

Stibel agrees that this is a significant factor. "Some ant species that develop complex social systems see similar reductions in individual brain size as the colony carries more of the cognitive load," he said. "We appear to have done something remarkably similar with culture and technology.

"What our research suggests is that we've undergone a fundamental shift in how cognition works," Stibel added. "Rather than relying solely on individual brainpower, we've become extraordinarily dependent on cultural and technological networks."

Stibel noted that large brains can be a burden, consuming more energy which requires people to eat more food to survive. "Large brains are metabolically expensive, consuming roughly 20% of our resting energy and producing significant heat." During times when food is scarce, such as during an Ice Age, this can result in people with larger brains being more likely to die of starvation.

This doesn't mean humans are smarter or dumber than they were; it just shows our intelligence is different.

"We've likely traded some raw computational capacity for the ability to leverage collective intelligence," Stibel said. "Whether that's a gain or a loss depends entirely on how you define intelligence. And, of course, how stable the cultural and technological systems we now depend on turn out to be in the long run."

See how much you know about the most complex organ in the human body with our brain quiz!

The samurai of Japan are famous for being skilled warriors who followed a code of honor. In popular culture and museums, samurai are often depicted as men, which raises a question: Were any samurai women?

Female samurai existed and there is some evidence that they fought in battle, several experts told Live Science. But how often they fought is a matter of debate, with some scholars calling it very rare and others suggesting it happened more often.

One important point in answering this question is that the samurai were an entire social class, sometimes called the "bushi" class. Anyone born into this class was a "samurai," regardless of whether they fought or practiced any form of martial arts.

"Any woman born in the samurai status group was a 'female samurai' even if she never picked up a weapon, just as any man born into that status group was a samurai, no matter how wimpy/untrained/etc. he may have been," Sean O'Reilly, a professor of Japan studies at Akita International University, told Live Science in an email.

It's unclear how often female samurai fought in battle, however. Women who fought in battle are sometimes called "onna-musha," which translates to "women warriors."

"I must say, as an historian, that onnamusha ‪—‬ female warriors ‪—‬ were probably not as frequent or as militarily significant as most people today believe," O'Reilly said.

Some particularly good evidence for female samurai participating in battle comes from the late 19th century, near the time when the samurai class was abolished, Diana Wright, who was a professor at Western Washington University, wrote in a 2001 article in the journal "War in History."

At that time, Japan was in a civil war as supporters of the Tokugawa Shogunate, which ruled Japan from around 1603 to 1868, battled those who wanted to overthrow the shogunate and return some of the military governor's powers to the emperor. The Boshin War, as it is sometimes called, lasted from January 1868 to June 1869. During that war, there were a number of recorded instances where female samurai, who fought on the side of the shogunate, engaged in battle, Wright noted in her article.

The shogunate forces were led by the Aizu domain (a regional government in northern Japan), and during the siege of the Aizu's capital of Aizu-Wakamatsu, a group of female samurai formed their own unit known as the "Joshigun."

"Although 20 to 30 women are believed to have made up the unit, the names of only 10 are known," Wright wrote. A 22 year-old woman named Nakano Takeko was the unit's unofficial leader.

Armed with only swords and naginatas (pole weapons with curved blades that can both stab and slash), they fought in a battle at Yanagi bridge against a force equipped with rifles, Wright explained. Records indicate that Nakano Takeko killed five or six men with her naginata before she was shot down. Ultimately, the battle ended in defeat and the surviving members of the Joshigun, along with the male troops, had to withdraw to a castle.

During the time of the Tokugawa shogunate, women of the samurai class were required to undergo martial art training with the naginata so they could defend themselves and their families, Wright noted. The amount of training they received varied, with the women of the Aizu domain tending to receive a larger amount.

Remains of female warriors?

A mound located in Numazu, a city in central Japan, may hold the remains of female samurai who fought in battle, some scholars believe. The mound contains human skulls, along with other skeletal bones, and an analysis of the mound's remains was published in Japanese in 1989 in the Journal of Anthropology. The skulls are from about 105 people; all of them were young adults when they died, and about one-third were women. They date to the 16th century, and scientists interpreted them as being the remains of people who were killed in combat, likely in the Battle of Senbonhama (also known as the Battle of Senbon Matsubara), which was fought between the Takeda and Hojo clans.

This mound is "indicative that women of fighting age fought and died in sixteenth century battles," Thomas Conlan, a professor of medieval Japanese history at Princeton University, told Live Science in an email.

However, Karl Friday, a professor emeritus of history at the University of Georgia, said the mound should be regarded with caution, as we can't be sure everyone buried in it actually fought in a battle. It's possible that some of the people buried in the mound were noncombatants who were killed anyway, Friday told Live Science in an email.

Stories and legends of female samurai

A number of stories refer to female samurai fighting in battle. Perhaps the most famous was Tomoe Gozen, who lived during the late 12th century. Stories say she served a lord named Minamoto no Yoshinaka and fought in the Genpei War, which was fought between the Taira and Minamoto clans between about 1180 and 1185, Thomas Lockley, a law professor at Nihon University who has studied and written extensively about the samurai, wrote in a 2022 article in the magazine Medieval World: Culture & Conflict.

One of the chronicles, called "The Tale of the Heike," says that as "a fighter she was a match for a thousand ordinary men, skilled in arms, able to bend the stoutest bow, on horseback or on foot, ever ready with her sword to confront any devil or god that came her way" (translation by Thomas Lockley).

Another famous woman mentioned in stories was Ōhōri Tsuruhime, who lived circa 1526 to 1543. She became the chief priestess of Ōyamazumi Shrine, located on the island of Ōmishima, after her father and brothers were killed while defending the island from a daimyo (a regional governor) named Ōuchi Yoshitaka, Stephen Turnbull, a historian who has written extensively on the samurai, wrote in his book "Samurai Women: 1187-1877" (Osprey Publishing, 2012). Despite being just 16 years old, she took charge of the island's defense force and defended it from the invaders. During her defense, she claimed to have been aided by the shrine's kami (spirit) and has been compared to Joan of Arc, Turnbull noted.

Some of what's said to be Tsuruhime's armor survives today and is displayed at the shrine. Conlan said that it is a suit of 16th century armor that is "tailored to the female anatomy."

However, Friday said we should be cautious when interpreting stories like these. "We do have stories about female warriors, like Tomoe Gozen, Hangaku Gozen, Ohori Tsuruhime, Ueno Tsuruhime, and a few others, but these women are all semi-legendary — especially with regard to their participation in battles," Friday told Live Science in an email.

Regardless of how accurate the stories are, female warriors became famous. "Mythologizing female warriors of yore began in Japan's Kamakura period [circa 1185 to 1333] and intensified in the Edo period [circa 1603 to 1868], with a huge proliferation of woodblock prints showing women holding naginata and so forth," O'Reilly said. Friday said the "very fact that these women became so famous is a pretty good indication of how uncommon female warriors must have been."

Taboos about women and battle

Friday thinks it would have been very rare for female samurai to engage in battle because it was considered taboo.

"One fascinating primer on military conduct, passed down within a branch of the Hōjō family, enjoined against such things as sharing quarters with women for three days prior to battles, allowing pregnant women or women who had recently given birth to touch a warrior's weapons, riding in boats with female passengers while in route to battle, and even allowing women to look upon the backs of officers departing for campaigns!" Friday said.

"The bottom line is that while there almost certainly must have been at least a few cases of women participating in Japanese battles over the course of [the] 8th to 16th centuries … there's absolutely no good evidence to support the conclusion that women warriors were any more common in Japan than they were in medieval France or ancient Sparta, much less that this occurred often enough to justify calling it a practice or even a phenomenon," Friday said.

While the samurai class was effectively abolished during the 1870s, some of the training practices done by female samurai are still carried out today. Eric Shahan, a Japanese translator who specializes in translating martial arts texts, noted that the Yoshin School (a branch of traditional Japanese martial arts) "still practices Naginata in Kimono, reflecting the fact that women may have to suddenly take up arms — and therefore have no time to change into training gear."

Can you identify these historical objects of war? Test your smarts with our weapons of the world quiz!

Whether it's having the "eye of the tiger" or being lionhearted, references to lions and tigers evoke power and determination. That's not surprising given these animals are both large, powerful felines. But they are distinct species. So what do these iconic cats have in common, and how are they different?

To answer this question, let's consider the two species' appearances, evolutionary histories and how they live their lives as kings of their ecosystems.

Appearance and anatomy

The most obvious difference between modern lions (Panthera leo) and tigers (Panthera tigris) is their distinct physical appearances, particularly their coats.

"Just looking at them, you can clearly tell that tigers have stripes and lions don't," Dell Guglielmo, a zookeeper who cares for lions and tigers at Smithsonian's National Zoo, told Live Science. "The lion males have the large huge mane, and the females do not." In contrast, male and female tigers look the same.

A tiger's stripes help the cat blend in among the patterns of shadows and light created by grasses and trees in the dense temperate and tropical forests of Asia where they live. The orange and brown or black stripes stand out to the human eye, but to a tiger's colorblind prey, orange appears green and blends in. The stripes are pigmented on their skin, too. Each tiger's stripe pattern is unique, Guglielmo said.

Lions' tawny coloring also serves as camouflage. Lions blend in with the dry grasses of Africa's open savannas.