My flight attendant told me I couldn't bring my pet goldfish on the plane.

This joke finds its humor swimming in the delightful pool of wordplay. The setup relies on the common understanding of "carry-on" as a piece of luggage that passengers bring into the plane cabin. The punchline, however, cleverly twists this meaning. The speaker is arguing that their goldfish is literally "carried on" board, making it a "carry-on" in the most basic, physical sense, rather than the airline-specific term for luggage. It's a classic play on words, where a familiar phrase is reinterpreted in an unexpected and silly way.

In the real world of air travel, bringing pets on a plane is a highly regulated affair. Airlines have specific policies for live animals, often requiring them to be in approved carriers, sometimes even traveling in cargo, and certainly not just tucked away like a handbag. The absurdity of trying to sneak a goldfish past a flight attendant by claiming it's a "carry-on" in the same vein as a suitcase adds to the comedic effect. It highlights the often rigid and sometimes illogical nature of travel rules, contrasted with a stubbornly literal interpretation that only a comedian could pull off.