Yawning is not something we usually aim to provoke among our readers, but have a yawn now. Does your brain feel cooler? Do you feel more attentive? According to psychologists Andrew Gallup and Gordon Gallup of the State University of New York at Albany, that is why we yawn: to boost blood flow and chill the brain.

Not only that, brain-cooling explains why you can "catch" a yawn, says Gordon Gallup. "We think contagious yawning is triggered by empathic mechanisms which function to maintain group vigilance." In other words, yawn-catching evolved to help raise the attentiveness of the whole group.

Sounds about right.