The landlady of a rooming house that had seen better days was leading a prospective tenant to a third floor room with badly splattered wall paper.
Landlady: “The last man who lived in this room was an inventor—he invented some type of explosive. “
Prospective tenant: “Then the spots on the wall were some type of explosive?”
Landlady: “No, the inventor. “
This joke is included in the Black Book of Jokes. You can order it from here.