A professor was seated next to a student. The student said to the professor,
"I'll ask you a question. If you don't know the answer, you should pay me $50.
Then you can ask me a question and if I don't know the answer, I'll pay you $5."
And then the student asked a very hard question.
The professor thought about it for 20 minutes but could not think of a solution.
He got out his wallet and paid the student $50. The professor said,
"Okay. What's the answer?"
The student did not say a word but got out his wallet and handed $5 to the professor.