An atheist was seated next to a Rabbi on an airplane and he turned to him and said, "Would you like to talk? Flights go quicker if you strike up a conversation with a fellow passenger."
The Rabbi, who had just started to read his sefer, replied to the total stranger, "What would you want to talk about?"
"Oh, I don't know," said the atheist. "How about how there is no God, or no Heaven or Hell, or no life after death?" as he smiled smugly.
"OK," said the Rabbi, "Those could be interesting topics but let me ask you a question first. A horse, a cow, and a deer all eat the same stuff - grass. Yet a deer excretes little pellets, a cow turns out a flat patty, and a horse produces clumps. Why do you suppose that is?"
The atheist, visibly surprised by the Rabbi's question, thinks about it and says, "Hmmm, I have no idea." To which the Rabbi replies, "Do you really feel qualified to discuss whether there is no God, Heaven or Hell, when you don't know shit?"
Needless to say, he was able to learn from his sefer uninterrupted for the duration of the flight.