Nobody knows why Olympic speedskater Apolo Ohno--or anyone else for that matter--yawns.

Here's what we do know about one of the strangest human behaviors:

Everyone yawns. 11-week old fetuses, babies, teenagers, adults, birds, reptiles and most mammals.

Dogs can catch human yawns. In one study, 21 of 29 dogs yawned after observing a human yawning. The dogs did not fall for the human's fake controlled mouth movements.

Yawns occur due to sleepiness, But they don't make us any more vigilant, according to this study. (Which means don't count on a yawn to keep you from falling asleep at the wheel.)

Six seconds. The lifespan of an average yawn.

Yawns are contagious. You can be infected by another person's yawn starting between the first and second years of life.

There are exceptions. Some research has shown contagious yawning is impaired in children on the autistic spectrum, which supports the claim that contagious yawning is based on the capacity for empathy.

Bored people yawn. Scientists compared the number of yawns in 17 to 19 year old students who watched music videos to the number of yawns in students who watched an uninteresting color test bar pattern. Guess who yawned more? 

Our ancestors yawned. Perhaps they did it to show off their teeth. Or maybe they used yawns as a way to coordinate social behavior: One person would yawn to signal an event and the others would yawn in response. But we still have no idea.

Popular but unproven yawning theory No. 1: We might yawn because our bodies are trying to get rid of extra carbon dioxide and to take in more oxygen. But then why does Ohno yawn before and not during his race? University of Maryland neuroscientist Robert Provine, a leading expert on yawning, questioned the physiological theory when he showed that giving people additional oxygen didn't decrease yawning and decreasing the amount of carbon dioxide didn't prevent it.

Popular but unproven yawning theory No. 2: Yawning cools the brain. Researchers found that people yawned more often when they pressed a warm or room temperature towel against their heads than when they pressed a cold towel against their heads. People who breathed through their noses (thought to reduce brain temperature) did not yawn at all.

Yawns have long been associated with stretching. In 1923, a British scientist showed that people paralyzed on one side of their body could move on their paralyzed side when they yawned.

It's not uncommon to yawn during sex. We often yawn at the moment we're shifting from one state to another, said Provine.

It's impossible to stop a yawn. Try this by clenching your jaw, sucking in air through your teeth or inhaling through your nose. Unsatisfying, isn't it?

Reading this piece will likely make you yawn. Just thinking about a yawn can trigger one.

http://featuresblogs.chicagotribune.com/features_julieshealthclub/2010/02/yawning-why-does-apolo-ohno-do-it-.html