Two economists are walking in the park. The first economist sees a pile of dog shit and says to the other, "I'll pay you $50 to eat that dog shit." So he does and gets paid $50. Later on, the second economist sees a pile of dog shit and says to the first, "I'll pay you $50 to eat that pile of dog shit." So he does and gets paid $50.

The first economist says, "I can't help but feel we just ate dog shit for nothing." "Nonsense," says the second economist, "We just contributed $100 to the economy."