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There is an interesting legend“knocking on the table”in drinking-tea custom. Once upon a time, the emperor Qianlong privately visited the folk, and drank tea with the local official Zhou riqing in a teahouse.
After pouring tea to himeself, the emperor also poured to Zhou riqing. Zhou riqing immediately felt extremely shocked, but did not dare to kneel down and thanked for the graciousness of the emperor, as he was worried about exposing the status of the emperor.
So what could he do? He quickly knocked on the table three times with two flexural fingers instead of kneeling down. Thus, this manner quickly spread in the folk and became a kind of thankful manner colloquially.
This is certainly a legend. Qianlong maybe never poured tea for himself in his life much less pouring tea to others.
Therefore, when someone pouring tea to the other, one should knock near the cup on the table which mean "thank you" and "please". It is a kind of politeness.