A student and an economics professor are walking across the quad. "Look, a $20 bill!", says the student. "Someone must have dropped it." "Nonsense!", replies

A student and an economics professor are walking across the quad. "Look, a $20 bill!", says the student. "Someone must have dropped it." "Nonsense!", replies the economics professor. "If there were a $20 on the ground, someone would have picked it up!"