A man walks into a bar and says “ow”
My informant overheard her roommate telling me a joke that started out with “a neutron walks into a bar…” and chimed in with this “walks into a bar” joke that she’d learned from one of her friends in high school.
This joke relies on the popularity of the “walks into a bar” structure. The joke works by using a familiar setup, but then switching the expected denotation of the word “bar.” From past experience with jokes based on this structure, the audience has been conditioned to expect that the man walks into an establishment which serves alcohol. Only after a moment of confusion does the audience realize that the “bar” in this joke uses a different definition of the word.