Why do you tap on the desktop to express your gratitude when others pour you tea?

This is because when others help pour tea, tapping on the desktop shows gratitude to the tea pourer, and it is a handshake ceremony. When the host pours tea for the guests, the guests can tap the desktop with their forefinger and middle finger to express their gratitude.

Use the word "hand" instead of "head", and the last two words are homophonic. In this way, "kowtowing" is replaced by "knocking hands", with three fingers bent to mean "three kneeling" and nine fingers tapped to mean "nine knocking". Drinking tea is still a common ceremony in China. Whenever the host asks for sugar to pour tea, the guests clap their hands to express their thanks.

Extended data:

There is a saying in old Beijing: "wine is full of respect, tea is full of gifts." When you suggest pouring wine, please fill it, but when you pour tea, please don't fill it. If you fill it up, you are driving away the guests. A further statement is: "tea seven, rice eight, wine ten." In other words, only 70% of tea can be poured, 8% of rice must be filled, and wine must be filled, which is 10%. Tea is poured to the point where it is seven minutes full; Eating at eight o'clock is called "touching your nose", which is disrespectful to the guests. On the other hand, exquisite guests generally don't "return to the bowl", that is, they don't add rice after a bowl.

Beijingers like to drink tea in a covered bowl, and there are many lectures here. When drinking tea from a bowl with a lid, hold the saucer and bowl in one hand, gently lift the lid with the other hand, and then hold it to your mouth for a sip. Never pick up the lid of the bowl, just like drinking water from a cup.

Baidu Encyclopedia-Hand Holding Ceremony