I do so love eavesdropping on people, but last night on the tube yielded my favourite exchange for a long time. Not intended as a joke, but it was structured like one, and it did make me laugh out loud. The more I think about it the more I like it. It makes no sense on so many levels:
Woman: Why do you think the terrorists set fire to King's Cross?
Man: I don't know
Woman: Because it's made of wood
You can turn this into a 'funny things you've overheard people saying' thread if you like.