"In a small town, an idiot breaks a shop window. He's called a vandal, until someone points out that a window installer now must be paid to replace the window. The window installer then will have enough money to buy a new suit. A tailor will then be able to buy a new desk. And so on. The whole town apparently gains from the economic activity generated by the broken window. Of course, if this made sense, cities should hire people to run though town, breaking windows.
But it doesn't make sense. It's a fallacy because the circulating money is seen; what is not seen is what would have been done with the money if the window were still whole. The shopkeeper, instead of paying the window installer, might have expanded his business, or bought a new suit or a new desk. The town is worse off because of a broken window."