See in 3D
Hey, wait a second... how do two eyes produce 3D vision?
It's actually a trick of the mind (or three tricks, to be exact). First, our brains utilize "binocular disparity" the slight difference between the images seen by our left and right eyes. Our brains use the two skewed versions of a scene to reconstruct its depth.
For a close-up object, the brain registers the "convergence" of our eyes, or the angle they swing through to focus on the object, to decide how far away it is.
When glancing at things on the go, we subconsciously gauge distance by registering "parallax." That's the difference in speed at which closer and farther objects seem to move as you pass them.
Get pins and needles
Hitting your funny bone isn't very funny. Neither is crossing your legs for too long, or waking up at night with a dead-seeming hand that gradually tingles back to life. What causes that horrible "pins and needles" sensation?
It happens when you apply too much extra weight or pressure on a nerve, temporarily inhibiting its function, and then remove the pressure. As the nerve gradually returns to normal, our brain somehow interprets its activity as a tingling sensation pins and needles.
Whether you're a woman battling armpit or pubic hair, or a man keeping a beard at bay, shaving is the norm for modern humans so much so that not shaving certain body regions strikes many people as unattractive. But strangely, this culturally-determined behavior defies our own evolution.
After all, armpit and pubic hair evolved to help trap pheromones. Considering that we exude those odorous chemicals for the sole purpose of attracting mates, it's odd that we now perceive their smell as pungent, and the hair that traps them, shave-worthy. Furthermore, beards evolved to help women distinguish men from boys; they indicate a man's maturity, and exaggerates his masculine jaw line. Nonetheless, today, most men opt for a clean-shaven look.
Risky endeavors can be deadly, as exemplified by the frequency of fatal sky-diving and mountain-climbing accidents. So, why don't we all just stay home and watch Netflix? Why this urge to do dangerous things?
Psychologists say risk-taking stems from our need to impress potential mates. Men face more intra-sexual competition than women, and so they must advertise their sexual fitness through daring exploits more overtly. This explains why men tend to take even more risks when they're in groups. Though women are generally more risk-averse, everyone strives to impress on occasion.
Yes, yes: everyone knows sex is how we reproduce. But why have two sexes in the first place, instead of a uniform population of asexually-reproducing individuals? After all, with twice the number of moms, populations of asexual organisms grow at double the rate of sexual organisms in lab experiments.
Biologists' best answer, called the Red Queen hypothesis, holds that organisms and the parasites that live on them are running a race in which they constantly evolve in response to each other's genetic mutations, maintaining an overall balance. Sex, the theory holds, gives host organisms a leg-up on this evolutionary treadmill by allowing two such organisms to shuffle their genes, creating new, rare combinations in their offspring and new challenges for coevolving parasites.
So, next time you're getting laid, thank the parasites within you.