What “mischief” does the speaker do?

The speaker’s mischief is that he challenges the neighbor’s unquestioned belief that “good fences make good neighbors.” He points out that their properties do not need a wall because there are no cows to contain, and asks what the wall is meant to keep in or out. So the best answer is “He questions the tradition of wall-mending.”

AI-written and checked against the verified answer.