Originally Posted by Doctor Diesel
My ex bird once tried to tell me that contagious yawning was caused by the vacuum created by the first yawn in the room, causing the second person's brain to realise that a greater intake of air was required
Do you often yawn when hoovering too?
Yawning is, apparently, a reflex that makes you show your teeth when you're tired - i.e. a show of aggression that says that "I might be tired by I'm still a threat".
Might be bollocks like, but it's more plausible than your 'vacuum' theory.