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!)
Also, although I often yawn when I see someone else yawning, this does not happen all the time (and I'm taking into account the fact that sometimes I yawned first). That is, I may not be yawning, I see someone else yawn, and it fails to provoke a sympathetic response in me. This happens mostly if I don't know the yawner personally, which may support the "hardwired ritual" theory mentioned in "Science Daily." I subconsciously do not recognize the stranger as part of my little "tribe," so his or her yawn carries less significance for me than if a friend or family member yawns.
So far, in my case, yawns seem to be connected with present (but not immediate) danger, and with some sort of social factor involved. I do yawn more when under stress than when relaxed. However, I also (though less frequently) yawn when relaxed and alone, so the conditions of danger and company are not requirements for yawning to occur.
It might be a vestigial function that served a more definite purpose in our distant past; if its pupose is more clearly defined in other species, that might help us figure it out. Dogs and cats yawn, and they are related to us only so far as they are placental mammals; we parted ways many millions of years ago. Somewhere, there may have been a common ancestor to whom yawning probably served a useful function that improved its chances of surviving & mating. Although the yawn has since been supplanted by other tactics (maybe an improved cardiovascular system), the yawn proved in its time to be so useful that it became an integral part of many species descended from this ancestor. Kind of like the appendix on the large intestine.