Excretes Jokes

Little Johnny was on a plane when the stranger in the next seat said, "Let's talk. Flights go quicker if you strike up a conversation with your fellow passengers."
Little Johnny, who had just opened his book, closed it slowly, and said to the stranger, "What would you like to discuss?"
The stranger said, "How about nuclear power?"
Johnny said, "That could be interesting. But first, I have a question. A horse, a cow, and a deer all eat the same thing, grass. Yet a deer excretes pellets, while a cow excretes a flat patty, and a horse excretes clumps of dried grass. Why do you suppose that is?"
The stranger said, "I have no idea."
Johnny said, "Well, then, why do you feel qualified to discuss nuclear power when you don't know shit?"