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Ban môn lộng phủ
| 班門弄斧 |
| (Literally) Demonstrating how to use an ax in front of Lỗ Ban. |
Note : Lỗ Ban was an ancient Chinese skilled carpenter, who is considered the master in this field.
The expression implies it is not recommended to teach people what they already know. This expression is similar to the English idiom:" Never offer to teach fish to swim".