Yep it is a mystery. Here are a couple of brief informative sites on the topic.
http://faculty.washington.edu/chudler/yawning.html http://www.nature.com/nsu/030728/030728-2.html http://www.msnbc.com/news/205574.asp http://kidshealth.org/kid/talk/qa/yawn.html

Ahhh Yawnnn,
Did you yawn yet?

[This message has been edited by Ascending1 (edited October 22, 2003).]