Never attempt to explain why a joke is funny. This is the only rule I swear by.

Here’s a great Jewish joke that only Jews will find funny:

Two rabbis are walking down the street, and they pass a cathedral. The cathedral has a sign in the lawn:
“Convert to Christianity - Get $10″. The rabbis look at each other. Rabbi Noah says to Rabbi David, “Well, should I check it out?” David replies “$10 for nothing? Of course!” So Noah walks into the Cathedral while David waits outside. 10 minutes pass. 20 minutes pass. Finally, after half an hour, Noah walks out. David impatiently asks him, “Well, did you get the money?”

“Is that all you people think about?”

Zing!