As retold by Bruce Lowitt

A visitor to New York stops at a window in the middle of which is one clock. He goes inside and says to the proprietor, a breaded man wearing a yarmulke, "Would you mind examining this watch. It's been running a little slow lately."

"Why are you asking me?"

"Because you're a jeweler or a watchmaker."

"No, I'm a mohel."

"What's that?"

"I do circumcisions."

"Then why do you have a clock in your window?"

Mister, what would you put in the window?"