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In the TV series, Shun Fang told a joke to the lawyer. What is that?

The first paragraph "Just kidding": One person plays the respondent and the other plays the questioner.

Son: Hey, Dad, listen to this. Knock, knock. (Fixed startup mode, indicating that this is a knocking game)

Father: Who's there? (Fixed question)

Son: Shelby. (You can answer casually: the names of ordinary people)

Father: Which Shelby? (Fixed answer)

Son: Shelby walked around the mountain when he came.