don't know shit

[Danimal]

Guy gets on a plane and finds himself seated next to a cute blonde. He immediately turns to her and makes his move.
"You know," he says, "I've heard that flights will go quicker if you strike up a conversation with your fellow passenger. So let's talk."
The blonde, who had just opened her book, closes it slowly and says to the guy, "What would you like to discuss?"
"Oh, I don't know," says the guy, smiling. "How about nuclear power?"
"OK," says the blonde. "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 the deer excretes little pellets, the cow turns out a flat patty, and the horse produces muffins of dried poop. Why do you suppose that is?"
The guy is dumbfounded. Finally he replies, "I haven't the slightest idea."
"So tell me," says the blonde, "How is it that you feel qualified to discuss nuclear power when you don't know shit?"