Two economists were walking down the street. One of them saw a $20 bill lying on the ground and reached down to pick it up.

The other economist, startled, said, "what are you doing?"

The first economist said, "I'm picking up that $20 bill."

The other economist was bemused. "There can't possibly be a $20 there to pick up. Why, someone would have already picked it up!"

The first economist straightened up, and considered carefully the words of his colleague. Convinced that it must be his imagination, he nodded, and the pair of them walked off.