What do you mean by teaching fish to swim?

Once upon a time, there was a man named Lu Ban, who was famous for his dexterity and was regarded as the ancestor of craftsmen. Experts admire him with admiration. Hearing this, the people next to him felt that he was too boastful and pointed to the Dahongmen behind him and said, "Ultrafine products, can you make a better door than this?" The young carpenter said proudly, "I'm not bragging." I tell you, I used to be a student of Luban. Can't I make such a simple door? It's a joke. " Everyone couldn't help laughing and said, "This is Mr. Luban's home. He made this door. Can you really make a better door than this? " The young carpenter ran away shyly. Chabudo, like me now, is "teaching fish to swim" here!