Confession and Tired Hands

Boy goes to Confession.

Boy: "What are you doing, father?"

Priest: "It's called masturbation and soon you will be doing it."

Boy: "Why do you say that, father?"

Priest: "'Cause my hand is getting tired."

Explanation

Experiment

This joke plays on the unexpected and inappropriate response from the priest. The boy is asking a genuine question about what the priest is doing, assuming it's some religious practice. The priest, instead of giving a proper answer, says it's masturbation and that the boy will soon be doing it too, then reveals he's just doing it himself. The humor is in the shock and the absurdity of the priest's confession and the punchline of his hand getting tired, which is pretty vulgar.