According to the explanation of the idiom dictionary, "moving the index finger" originally refers to the omen of something delicious to eat, and later it describes the appearance of being greedy when seeing something delicious. The most original source seems to be in "Zuo Zhuan·Xuan Gong 4th Year": "The people of Chu offered turtles to Duke Lingying of Zheng. The prince of the Song Dynasty was about to meet with the Zi family, and the prince's index finger moved." The specific story is as follows: In the Spring and Autumn Period, Chu The people of the country presented a turtle to Zheng Linggong, the king of Zheng. When Song and his family were preparing to enter the palace, Song's index finger suddenly moved on its own. He showed it to his family and said that according to his The experience is that if a person moves his index finger, he will surely have a good feast. After they entered the palace, they saw the cook killing the turtle. The two of them laughed knowingly. Zheng Linggong asked them why they were laughing, and the young master told the truth. Maybe he wanted to make a joke. When it came time to eat turtle meat, Duke Linggong of Zheng deliberately refused to let his son Song eat it, which made him very angry. He dipped his finger in the cauldron and tasted some of the juice before leaving the palace. Zheng Linggong was very angry and wanted to kill his son Song, but he and his family took the initiative and murdered him. When you see this, you know that this story may only let us know it but not know the reason. The index finger belongs to the young master Song, and it discovered that unexpected food will spontaneously activate. That's it. If you want to ask why your index finger moves when it touches food, or why it doesn't move elsewhere, I think Mr. Song himself can't explain it. But I don’t know if psychology, neurology or sports science can explain it clearly now. It may be that some people have extremely sharp nerves and may detect something wrong in advance when encountering danger and avoid it. This may be attributed to some type of psychological reaction and conditioned reflex.