A university student delivers a pizza to an old man's house.

"I suppose you want a tip?" says the old man. 

"That would be great," says the student, "but the other guy who does deliveries told me not to expect too much – he said if I got 50p, I'd be lucky." 

The old man looks hurt. "Well, to prove him wrong, here's £5. What are you studying?"

"Applied psychology," replies the student