Johnny paid his way through college by waitering in a restaurant.

"What's the usual tip?" asked a customer.

"Well," said Johnny, "this is my first day, but the other guys said that, if I got five dollars out of you, I'd be doing great."

"Is that so?" growled the customer. "In that case, here's twenty dollars."

"Thanks. I'll put it in my college fund," Johnny said.

"By the way, what are you studying?" asked the customer.

"Applied psychology."

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Edit: This is not my joke