Why did the fox suddenly tell a joke to the sloth in Zootopia?

I didn't know until I saw the movie. On the surface, the fox was just playing cheap and for fun. Another reason is that the fox was forced by the recording to help Judy. He was upset and retaliated against Judy. Sadism is relieved when Judy is nervous; This also reflects the sloth's slowness and inefficiency.

This stalk is a successful joke, and the audience in the cinema are very happy.