There was an economist who was walking down the street with a friend. The friend stops and says, "Look, there is a $20 bill on the ground!" The economist turn

There was an economist who was walking down the street with a friend. The friend stops and says, "Look, there is a $20 bill on the ground!" The economist turns and coolly replies, "Can't be. If there was a $20 bill on the ground, somebody would have already picked it up."