Why r yawns contagious?

Why r yawns contagious?
I just read the reason why in Malcom Glidewells book "The Tipping Point (How little things make a big difference). He states that yawns are incredibly contagious. If you see someone yawn or even hear it, you will yawn. He also states they are emotinally contagious. (I just re-read the section on it and that's all that's mentioned about it). If you get the chance, read the book - it's a great read!
Wow. Ya learn something new everyday. I'm not sure!
The same reason you wasted pionts on this question! Yawn, Yawn!
because there is not snough oxigen in the room
Monkey see, Monkey do.
Because deep yearns for deep. Or something like that. I have read how it really happens but cannot remember.
someone told me once that it was due to he air you breathed out... (carbon dioxide), however not too sure how accurate that is!!
my gf is the only one that can make me yawn by her yawning. silly me, I thought it worked both ways. I thought I was the only one that could make her yawn, but I recently found out this was not the case-anyone can make her yawn. I felt wierd about that ,almost but not quite betrayed LOL.. anyway I am sure there' a scientific reason but to me its just magic how that works.
We're all tired.....yawning reminds us how much. lol
Yawnnnn I know the answer and can't stop yawning now to type it all Yawnnnnn Apparently even the mention of yawning gets to me YAWWWNNNNNN