I thought it was to sort out inner ear pressures (by opening the Eustachian tube at the back of your throat) so you can hear a bit more clearly.
Which doubles as a nice explanation of why it's contagious - one person yawns, which sorts him out but changes the air pressure for everyone else (nb geeky joke)
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I can't understand why people are frightened of new ideas. I'm frightened of the old ones. -- John Cage (1912 - 1992)
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