A traveller in a strange town found that his watch stopped.

He happened to see a door with a big clock hanging outside, so he stepped in.

An elderly Jewish man attended him and he explained that his watch needed fixing.

The elderly man said I am sorry, sir I cannot help you, I am a mohel not a watchmaker

The man replied, surprised If you are a mohel why do you hang a clock outside your door? The mohel replied Well, if you were a mohel, what would YOU hang outside your door?