A man asked an old man his last name. He said his surname was Zhang. Who knows this joke?

In class, I was translating the classical Chinese into a sentence: "Dongpo holds the tail in his right hand and caresses Lu Zhi's back with his left hand."

Su Shi held one end of the scroll in his right hand and touched Lu Zhi's back with his left hand. The teacher paused.

Youyou said, "What is the relationship between Su Shi and Lu Zhi?"