What does the donkey of Zhu Gejin's son Yu Zi mean in Romance of the Three Kingdoms?

In the Three Kingdoms, our general was called Yu. He has a son named Zhuge Ke, who has been smart and eloquent since childhood. Sun Quan loves this child very much.

On one occasion, Sun Quan gave a banquet in the imperial court, and six-year-old Zhuge Ke attended with his father. Zhu Gejin's face is so long that Sun Quan wants to make fun of him. He got drunk, ordered a donkey, and wrote the words "Zhuge Yuzi" on the donkey's long face, so as to ridicule Zhu Gejin's face, which looks like a donkey. They laughed when they saw it, and Zhu Gejin felt very embarrassed. When Zhuge Ke saw this, he went to Sun Quan's seat and knelt down to let him add a word. Sun Quan ordered someone to bring him the pen. Zhuge Ke added the word "donkey" after "Zhuge Yuzi" and became "Zhuge Yuzi's donkey". The full house of ministers was amazed. Sun Quan was very happy to see Zhuge Ke so alert and gave him the donkey on the spot.