Surprisingly little is understood about yawning. Here's what we do know:

• all animals yawn
• most yawns occur during the hour before sleeping and the hour after waking
• seeing a picture of someone yawn makes you want to yawn too
• watching a movie of a yawn is even more powerful
• no single feature of the face (e.g., mouth only) is as good as the whole face
• yawning is more contagious when people don't know they're being watched
• yawning is generally believed to help arouse the drowsy person (so they inhale more air and get more oxygen to the brain) though some believe it just helps communicate how drowsy someone feels

One theory explaining the "why" of yawning is that yawning is a "group signal" persisting from humanity's early days of hunting in groups. All the group members needed to sleep at the same time in order to ensure all got a good rest. The yawn signaled to others that the time to sleep had arrived.