Hehe... on a plane

A stranger was seated next to a little girl on an airplane. When the plane took off and settled into its climb, the stranger turned to the little girl and said: 'I've always found 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, 'OK. What would you like to talk about?'
Oh, I don't know,' said the stranger. 'How about nuclear power?'
'Yes,' 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 grass -- the same stuff. 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 thought for a few moments, then said: 'You know, I've never thought about that. I have no idea.'
The little girl began to open her book again, saying: 'Do you really feel qualified to discuss nuclear power when you don't know shit?'