How much does a chimney cost?

The joke “How much does a chimney cost? Nothing! It’s on the house” is a play on words that exploits the double meaning of the phrase “on the house.” In common parlance, “on the house” means something is free or complimentary, usually in the context of a bar or restaurant. In the literal sense, a chimney is a structure that is physically “on” a house, as it protrudes from the roof. The joke combines these two meanings to create a humorous answer to the question. When asked how much a chimney costs, the punchline “Nothing! It’s on the house” cleverly conflates these interpretations, suggesting both that the chimney is free and that it is a part of the house’s structure.