Would you believe it that till today, scientists still don't know why we yawn!
From wikipedia, here's a list of superstitions that surround the act of yawning:
The most common of these is the belief that it is necessary to cover one's mouth when one is yawning in order to prevent one's soul from escaping the body. The Ancient Greeks believed that yawning was not a sign of boredom, but that a person's soul was trying to escape from its body, so that it may rest with the gods in the skies. This belief was also shared by the Maya.
Other superstitions include:
* A yawn is a sign that danger is near.
* Counting a person's teeth robs them of one year of life for every tooth counted. This is why some people cover their mouths when they laugh, smile, or yawn.
* If two persons are seen to yawn one after the other, it is said that the one who yawned last bears no malice towards the one who yawned first.
* The one who yawns first shows no malice towards those he or she yawns around.
* If you don't cover your mouth while yawning, then the devil will come and rob your soul (Estonia).
* In some Latin American, East Asian and Central African countries yawning is said to be caused by someone else talking about you.
* A yawn may be a sign that one is afflicted by the evil eye (Greece).
* When one person yawns, it is said that anybody watching will instantly yawn as well
Link [wiki] - Thanks Erik Olson (you're right - it was impossible to read the article without yawning!)