Two economists are walking down the street and see a $20 bill lying on the sidewalk. The first economist says, "Look at that $20 bill." The second says, "That

Two economists are walking down the street and see a $20 bill lying on the sidewalk. The first economist says, "Look at that $20 bill." The second says, "That can't really be a $20 bill lying there, because if it were, someone would have picked it up already." They walk on.