Two economists are walking down Constitution Avenue after the 4th of July Parade, when they come upon a pile of horseshit. The first economist says to the second economist, "I'll give you twenty thousand dollars if you eat that." The second economist runs his optimization calculations and decides that, yes, he will be better off; so he eats the horseshit and takes the money. They walk along a little farther and they come upon another pile of horseshit. Now the second economist - with twenty thousand dollars in his pocket - turns to the first economist and says, "I'll give YOU twenty thousand dollars if you eat that." So the first economist eats the horseshit and takes his money back. They walk a while in silence until the first economist says to the second, "We both have the same amount of money that we started with, but now we've both eaten a pile of horseshit." To which the second economist replies, "Yes, that's true; but you're overlooking the fact that GDP just grew by Forty Thousand Dollars!"
Paraphrased from a podcast by NPR's Planet Money