A prime example of an inefficient market

Everyone's heard the old saw:
An economics professor and a grad student are walking along the sidewalk, and the grad student spots a twenty dollar bill on the sidewalk. He says, “Hey professor, look, a twenty dollar bill.” The professor says, “Nonsense. If there were a twenty dollar bill on the street, someone would have picked it up already.”

I found an honest-to-god, non-metaphorical twenty dollar bill on the ground today. I picked it up.