Darn, I'm missing the reason...
Old joke.
A guy is walking down the street on the Lower East Side and sees a shop with a grandfather clock in the front window. "Oh, a watchmaker! I need to fix my watch." So he walks in and asks the balabos if he can fix his watch. "Fix your watch? No, I'm a mohel." "A mohel? Then why do you have a grandfather clock in the front window?" "Nu, tell me, if you were me, what would YOU put in the front window?"