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?