I admit to sometimes forgetting to cover my gaping yap when I yawn when it's just me and the Bean. She finds it terribly entertaining. So much so, that sometimes I will fake a yawn just to get a smile. Then it occurred to me that unlike adults, she doesn't yawn in response. And I began to wonder at what age babies/children develop the reactive yawn reflex. So I did a little research (god bless the internet) and it turns out that yawns become contagious to babies between one and two years of life.
Interesting, n'est-ce pas?
I also found out that one theory behind the yawn is that in caveman days the leader of the pack (herd? clan?) would yawn and this signaled to everyone else that it was sleepytime. Therefore, yawns are an unneccessary remnant of those early days since we have clocks now and don't need our leaders to tell us when to sleep. Which is good because I wouldn't even trust most of our leaders to tell me when to brush my teeth.
There are lots of other theories behind the need to yawn, including oxygen deprivation, the need to stretch facial muscles and my favorite, that our brains are overheating. I'm sticking with the caveman theory, though.