There was a recent study that suggested we yawn to cool our brains as a means to keep us alert. And that the reason yawns are contagious is that it was a way to keep a group alert and aware of potential dangers.

http://www.sciencedaily.com/releases/2007/06/070621161826.htm

"Science Daily — The next time you "catch a yawn" from someone across the room, you're not copying their sleepiness, you're participating in an ancient, hardwired ritual that might have evolved to help groups stay alert as a means of detecting danger. That's the conclusion of University at Albany researchers Andrew C. Gallup and Gordon G. Gallup, Jr. in a study outlined in the May 2007 issue in Evolutionary Psychology (Volume 5.1., 2007).

The psychologists, who studied yawning in college students, concluded that people do not yawn because they need oxygen, since experiments show that raising or lowering oxygen and carbon dioxide in the blood fails to produce the reaction. Rather, yawning acts as a brain-cooling mechanism. The brain burns up to a third of the calories we consume, and as a consequence generates heat.

According to Gallup and Gallup, our brains, not unlike computers, operate more efficiently when cool, and yawning enhances the brain’s functioning by increasing blood flow and drawing in cooler air."

And Gawker just posted this on Friday: a link to a picture of a 95-year-old grandmother with a horn growing out of her head:

http://zmadison.blogspot.com/2007/05/yes-thats-horn-growing-out-of-her.html