You Don't Know Shit Joke

A stranger was seated next to a little girl on an aeroplane when the stranger turned to her and said 'let's talk. I've heard flights go quicker if you strike up a conversation with your fellow passenger'
The little girl, who had opened a 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 first let me ask you a question. A horse, a cow and a deer all eat the same stuff - 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, visibly surprised by the little girls intelligence, thinks about it and says 'hmm 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?"