Why do we yawn?

Why do we yawn?
every time George Bush says something stupid, somebody yawns
a withdrawal of oxygen to our brains?
I've read that it's to keep adjectives members of the pride, flock, pack, (insert group of mammals here) on the same time agenda.
Yawning says "Okay, guys, it's time to rest", so adjectives rest.
Basically, the science report said it was to maintain everybody doing pretty much the same entry so that the group stayed together.

My logic says it's because of shallow breathing.
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