A good one...

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?