But the most remarkable characteristic of a yawn is that it is contagious. Viewing a yawning face with the mouth hidden will still stimulate a yawn for another person, and even thinking about a yawn starts the whole process, according to the encyclopedia. Stretching the jaws is apparently vital to completing a 'satisfactory' yawn, according to the encyclopedia. Stretching does not necessarily accompany yawning, but a yawn always accompanies a stretch, according to the encyclopedia. A yawn, which is involuntary and usually finishes itself, averages about six seconds and is a pleasurable experience for most people, according the encyclopedia.
Some people are more visual about their yawning.
But some people don't appreciate other people making them yawn.