The human race has scaled the tallest mountains, charted the deepest oceans and played a quick front nine on the freaking moon, but there's one frontier that still largely mystifies us: our own bodies.
There are everyday phenomenons you'd think must have been explained ages ago, but in reality asking these simple questions of a scientist will net you at best a shrug, and at worst some bullshit he just made up off the top of his head.
The act of yawning is baffling to experts for two reasons. One, it doesn't actually seem to serve any purpose. Seriously, when you feel a yawn coming on, suppress it. What happens? Do you go into convulsions? Is your face racked by pain? Does blood shoot from your nose? No. Not a damned thing happens.
Equally baffling, though, is the contagious nature of it. Yawn, and whoever sees you will yawn. When a chimpanzee yawns, the other chimps yawn. If you yawn, you can make a dog yawn. Seriously, try it.
Odds are you've yawned once just because you read the word "yawn" several times above. Why?
Science's Wild-Ass Guess:
Your science textbook in elementary school may have said that low oxygen levels in the blood triggered yawning, with the yawn providing a quick influx of the gas. That was the prevailing theory going back to the days of ancient Greece. As is usually the case though, it turns out people from back in the day didn't know what the hell they were talking about. In fact it's been found yawning may actually decrease oxygen intake. Makes sense, when you do hard exercise you don't start frantically yawning. You don't see athletes yawning in the middle of a sprint.
Unfortunately, the alternatives are quite a bit more insane.
Such as the theory that yawning is the body's way of controlling brain temperature. Yeah, apparently scientists think our brains function with all the complexity of an old car engine. And you know how you're always yawning when you wear a hat, right? Right?
The proof of this was experiments in which it was found people with cool packs attached to their heads yawned less. Unless there could be some other reason people sitting in an unfamiliar lab with ice packs on their heads weren't much in a yawning mood...
As for why yawning is contagious, some scientists have pointed to human being's primitive herd instincts, figuring group yawning could have helped regulate sleeping patterns so that a "whoops, we all fell asleep at once and got eaten by giant sloths" situation didn't develop.
This remains merely a theory though, and of course still doesn't explain why people yawn while on their own.