Teaching Lu Ban How to Suck Eggs 班門弄斧

The Chinese idiom 班門弄斧 (ban1 men2 nong4 fu3) literally means the ordinary craftsman is showing off his skills with the axe in front of Lu Ban, the master carpenter.

The origin of the idiom can be traced to a poet, Li Bai, of the Tang Dynasty (701-762 AD). He was a very gifted poet and following his death, the people buried him in a beautiful tomb on the Caishi River. Everyday admirers came to visit the tomb and many wrote poetry on the tombstone. One day during the Ming Dynasty (1368-1644) a scholar came to visit Li Bai's tomb and was appalled to see that it was covered with poorly written poems. He decided to add his own poem to the tombstone to stop future visitors from writing any more. The poem said

A tomb near Caishi River marks the everlasting fame of Li Bai; to and fro, every passer-by writes a poem on the tombstone just like a poor carpenter trying to show his proficiency with the axe before Lu Ban.

The equivalent English idiom is try to teach one's grandmother how to suck eggs. It is possible that this expression alludes to the fact that a toothless grandmother will be naturally more successful in sucking the contents from an egg than a grandchild with a complete set of teeth. Both the English and Chinese expressions are used to either: a) ridicule or reprimand someone who tries to show off in front of an expert; or b) to express one's modesty when demonstrating a skill in front of colleagues.