What does this couplet by Ji Xiaolan mean?

"Master and Disciple" versus "Father and Son"

In the Qing Dynasty, Ji Yun, also known as Xiaolan, served as the chief compiler of the Sikuquan Library. . One day, his teacher invited him to have a drink. There were two father and son at the table, both of whom were ranked Chinese candidates in the imperial examination in the year of Wuzi (1768). After drinking for a few rounds, the teacher who was in high spirits suddenly said to Ji Yun: "Xiaolan, you are very good at pairing. Now I will make the first couplet. If you can improvise the second couplet, I will give you a hundred gold ancient inkstone as a gift." , If you don't get along, I'll drink three glasses of wine as a fine!" Ji Yun smiled and nodded in agreement.

So, the teacher pointed to the two father and son who were on the same list and said: "Father and son, son and son, father and son and son." As soon as the first couplet was spoken, other guests present immediately felt that this second couplet was extremely difficult. Yes. Because "father and son" and "武子" have the same lower characters; the upper character is "father" and the other is "五". Although they are not the same characters, they are homophones ("father" is pronounced fù, and "武" has the ancient sound mào, but in the Qing Dynasty Pronounced as wǔ (sound). There are two characters for "father", three for "wu" and five characters for "son" in the whole sentence. It is really difficult to get the word "son" right and correct!

However, no matter how difficult the pair is, it cannot defeat Ji Yun. He glanced at the teacher and immediately thought of the official positions of the master and the apprentice: the teacher is currently the Minister of Household Affairs, and he is currently the Minister of Household Affairs. Therefore, later generations also commonly referred to the Minister of Household Affairs and the Minister as Situ. Then a wonderful second line suddenly burst out: "Master Situ, Tu Situ, Master Situ."

The whole hall burst into applause. The cheerful teacher personally delivered the ancient inkstone worth a hundred gold to Ji Yun's hands.