I used to be like this, and I didn't know I was like this until I was asked by my friend at dinner. Later, I thought it was probably because some rice grains stuck to chopsticks when I was eating, and I was afraid that it would fall on the dish when I was picking it up. I felt very rude, so I knocked on the bowl, making my heart think that the rice had fallen into the bowl, making my subconscious feel better, and then I went to get the food. Maybe so.