Wise Kid

A man was seated next to a kid in an airplane. The man turned to him and said, "Let's talk".
KID: Ok, what do we talk about?
MAN [making fun of the kid]: How about nuclear power?
KID: Very interesting topic. But let me ask you a question. Horse, cow & deer, all eat grass. Yet deer excretes pellets, cow flat potty and horse clumps. Why?
MAN: I don't know.
KID: Do you really feel qualified enough to discuss nuclear issues when you don't know shit!?