Why do we yawn, and why is it contagious?

The honest answer is that nobody knows for sure. Scholars have put forward about twenty reasons for yawning and still don't agree on which one matters most. The best-known explanation, that you yawn to grab extra oxygen, doesn't hold up well: giving people more oxygen doesn't make them yawn less. Two more promising ideas are that a yawn helps cool the brain, and that it helps you stay alert.

A yawn is a reflex in three acts: a long breath in as the mouth slowly gapes, a brief peak where muscles stretch, then a quick breath out as everything relaxes. At its peak, your throat and lungs open up to three or four times their usual size, and a tiny muscle in your middle ear tightens, which is why you hear that rumble inside your head.

Then there's the contagious part. Seeing someone yawn, reading about yawning, or even thinking about it can set you off. People noticed this long ago: in 1508 Erasmus wrote that one man's yawning makes another yawn, and a French proverb claims one good yawner sets off seven more. Modern studies find yawns spread most easily between relatives, then friends, and least between strangers. Even dogs catch our yawns. Whether that means contagious yawning is a sign of empathy is still hotly debated. Have you yawned yet?