Confucius had many students. It was said that he had more than three thousands students, seventy two of who were capable and virtuous gentlemen. Of course, there were also some bad students. Zai Yu was one of them.
Zai Yu was a good-looking man from state Lu, the home state of Confucius. He was a facile speaker, which impressed Confucius so deeply that the sage even expected him to become a better scholar than Zi Gong, a famous student of Confucius's, at their first meeting. In later years, however, Zai Yu gradually showed his true color: lack of moral and love, and lazy (He slept while other student studying hard).
Even a great educator like Confucius did not know how to teach him. After several times of failure of instructing, Confucius made the famous comment used by later teaches for thousands of years on Zai Yu: "You cannot carve on the mouldered wood", that is, Fools grow without watering in English saying.
Another mistake that Confucius made on judging students was that he overestimated Zi Yu, a student also from state Lu, whose name was Zhan Tai Mei Ming and Courtesy name was Zi Yu, and who was 39 years younger than Confucius.
When Zi Yu came to Confucius and asked to be a student of his, the sage, seeing the ugly face and body of Zi Yu, thought that he had no gift to be a good scholar. Yet, to Confucius's surprise and delight, Zi Yu studied extremely hard and was very good at applying knowledge that he learned from his teacher to real life. At the end of three years, because of his diligence, he exceeded his classmates in both his moral and academic accomplishments.
After returning to his hometown, he soon won the respect of people from all walks of life. When he was invited to give lectures in state Wu, more than three hundreds students followed him everywhere he went. His name was even heard by the rulers of the most influential states.
Learning of this, Confucius was euphoric about his student, but at the same time felt sorry for making a flawed assessment of him at first. He often said to his other students:" Judging a talent by his speech, I was wrong on Zai Yu; Judging a talent by appearance, I am wrong on Zi Yu ".
Background and Writer Comment:
The idiom of Judge A Person By His Appearance--Yi Mao Qu Ren has the same meaning and usage as its English counterpart: "Judge a book by its cover".