发布时间：2016-03-30 来源：新东方在线 发布人：zolo
Yawning is a puzzling phenomenon. Even the simple act of talking about it is enough to make some people do it (some of you are probably doing it right now). There really is no other bodily function quite like it.
Now, some of you reading this may be aware of the long-standing theory that the purpose of yawning is to keep us alert by forcing our bodies to take in an extra large gulp of oxygen. That makes sense, because we mostly yawn when we're tired or bored, situations where an extra burst of energy would come in handy.
The thing is, experiments have conclusively disproven that theory over the years. In fact, there is no universally agreed upon theory for why we actually yawn, even though everyone does it. A commonly accepted theory is that yawning actually cools down the brain, because various experiments have shown that one of the few things to actually change in the body during a yawn is the temperature of the brain itself.
As for why yawning is contagious, no one knows that either.