Fission or Fusion?

A stranger was seated next to a little girl on an airplane when he 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?"

"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 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 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?"