Slide 1 of 15
Ask a physicist about the radius of the black hole at the center of the galaxy and she'll tell you more than you wanted to know. Ask her how a bicycle works, and she'll shrug. It may surprise you to learn that scientists lack explanations for some of the simplest questions you might think to ask. Read on for a taste of the many seemingly mundane questions no know can answer.
Why do cats purr?Slide 2 of 15
Why do cats purr?
From house cats to cheetahs, most felid species produce a "purr-like" vocalization, according to University of California, Davis, veterinary professor Leslie Lyons. Domestic cats purr in a range of situations — while they nurse their kittens, when they are pet by humans, and even when they're stressed out. Yes, you read right: Cats purr both when they're happy and when they're miserable. That has made figuring out the function of purring an uphill struggle for scientists.
One possibility is that it promotes bone growth, Lyons explained in Scientific American. Purring contains sound frequencies within the 25- to 150-Hertz range, and sounds in this range have been shown to improve bone density and promote healing. Because cats conserve energy by sleeping for long periods of time, purring may be a low-energy mechanism to keep muscles and bones healthy without actually using them. However, this tentative theory doesn't explain why cats purr in the situations they do. "I am pretty sure this one will stay a mystery still cannot get cats to talk about it no matter how hard I try," Lyons told Life's Little Mysteries. [10 Facts For Cat Lovers]Slide 3 of 15
How do bicycles work?Slide 4 of 15
How do bicycles work?
We've been riding them for about a century, all the while thinking someone out there had a handle on how, exactly, they worked. But as it turns out, no one did. And they still don't.
Bikes can stay upright all by themselves, as long as they're moving forward; it's because any time a moving bike starts to lean, its steering axis (the pole attached to the handlebars) turns the other way, tilting the bike upright again. This restorative effect was long believed to result from a law of physics called the conservation of angular momentum: When the bike wobbles, the axis perpendicular to its wheels' spinning direction threatens to change, and the bike self-corrects in order to "conserve" the direction of that axis. In other words, the bike is a gyroscope. Additionally, the "trail effect" was thought to help keep bikes stable: Because the steering axis hits the ground slightly in front of the ground contact point of the front wheel, the wheel is forced to trail the steering of the handlebars.
But recently, a group of engineers led by Andy Ruina of Cornell University upturned this theory of bicycle locomotion. Their investigation, detailed in a 2011 article in the journal Science, showed that neither gyroscopic nor trail effects were necessary for a bike to work. To prove it, the engineers built a custom bicycle which could take advantage of neither effect. The bike was designed so that each of its wheels rotated a second wheel above it in the opposite direction. That way, the spinning of the wheels canceled out and the bike's total angular momentum was zero, erasing the influence of gyroscopic effects on the bike's stability. The custom bike's ground contact point was also positioned in front of its steering axis, destroying the trail effect. And yet, the bike worked.
The engineers know why: they added masses to the bike in choice places to enable gravity to cause the bike to self-steer. But the work showed there are many effects that go into the stability of bicycles — including gyroscopic and trail effects in the case of bikes that have them — which interact in extremely complex ways.
"The complex interactions have not been worked out. My suspicion is that we will never come to grips with them, but I don't know that for sure," Ruina told Life's Little Mysteries.Slide 5 of 15
Why does lightning happen?Slide 6 of 15
Why does lightning happen?
We know why lightning strikes: It happens because positive electric charges build up near the tops of thunderclouds and negative charges build up at the bottoms. The electrical attraction between these opposite charges, and between the negative charges and positive charges that build up on the ground below, eventually grows strong enough to overcome the air's resistance to electrical flow. The charges suddenly shoot toward one another and connect, completing an electrical circuit and triggering a flash of "lightning" as charges shoot along the circuit they have formed.
But why do opposite charges build up in different parts of clouds? [Gallery of the Craziest Clouds]
It's a subject of great theoretical debate. One theory holds that when ice particles within a cloud collide, they tend to fracture into smaller particles with positive charge, and larger particles with negative charge. Gravity pulls the larger, negatively charged particles downward, and updrafts lift the smaller, positively charged particles upward, resulting in an imbalance. But measured values of electric fields in thunderclouds don't seem to match those scientists woudl expect to result from this process. Another theory holds that high-energy electrons delivered by cosmic rays from space shoot down through the cloud, stripping off more negatively charged electrons as they go and dragging them toward the bottom of the cloud, causing the charge imbalance. Which is the right explanation? The jury of lightning scientists is still out.Slide 7 of 15
Why are moths drawn to lights?Slide 8 of 15