An American gets on a plane and finds himself seated next to Irishman.  He immediately turns to Irishman and makes his move.

"You know," says the American, "I've heard that flights will go quicker if you strike up a conversation with your fellow passenger.  So let's talk."

Irishman, who had just opened his book, closes it slowly and says to  the American guy, "What would you like to discuss?"

 "Oh, I don't know," says the guy, smiling. "How about nuclear power?"

"OK," says Irishman. "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 American guy is dumbfounded. Finally he replies, "I haven't the  slightest idea."

"So tell me," says Irishman, "How is it that you feel qualified to  discuss nuclear power when you don't know shit?`"