A carpenter in Chelm is fixing the roof, but as he works, he throws away about half the nails.
The mayor is passing by, and asks him why he's wasting so many nails. The carpenter answers,
"I take a nail out of the bag, and if it's facing the roof, I use it; if it's facing away, I know it's defective and throw it away."
The mayor tells him, "You fool! Those are for the other side!!"