There are many such scenes on TV, "Count your fingers". What is this? How to calculate?

It seems mysterious to show the fortune teller "Let me count my fingers". In fact, this is just a way to calculate heavenly stems and earthly branches:

Generally, a pen and a piece of paper are needed to cooperate with the ten-day stem and the twelve-day stem, but the ancients rarely had the opportunity to use it when calculating numerology. So just divide your fingers into three sections and fix the twelve-day stems. When matching with the stems, you need to read the stems according to your knuckles, so that you can quickly calculate the heavenly stems and earthly branches of the year and month, and you can get it when you pinch your fingers.