Here's One I Hadn't Heard Before.

Via email from a family friend:

A stranger was seated next to a little girl on the airplane when the stranger turned to her and said, 'Let's talk. I've heard that flights go quicker if you strike up a conversation with your fellow passenger.'

The little girl, who had just opened her book, closed it slowly and said to the stranger, 'What would you like to talk about?'

'Oh, I don't know,' said the stranger. 'How about nuclear power?' and he smiles.

'OK,' she said. 'That could be an interesting topic. But let me ask you a question first. A horse, a cow, and a deer all eat the same stuff - grass - yet a deer excretes little pellets, while a cow turns out a flat patty, and a horse produces clumps of dried grass. Why do you suppose that is?'

The stranger, visibly surprised by the little girl's intelligence, thinks about it and says, 'Hmmm, I have no idea.'

To which the little girl replies, 'Do you really feel qualified to discuss nuclear power when you don't know shit?'