I won't take offense if this article makes you look a bit sleepy. Just reading about yawns can — wait for it — induce one without warning. And it’s not because your brain needs oxygen, whatever the schoolyard myth.
Some current theories focus on yawning as a social cue (like laughing) that communicates information and helps organize group behavior. By passing from person to person with unconscious ease, a yawn can convey some piece of information (perhaps "I’m tired", or "this opera is dull") around a social circle. Such a tool would have come in handy in pre-language days. But does the science back this up?
One 2005 study scanned the brains of people watching yawns, and located unique activity patterns in brain areas related to behavioral mimicry. So next time you get a dirty look for yawning in a meeting, chalk it up to caveman communication.