Why do fortune tellers only need birthdays?

What is the theory of fortune telling? Why does he sometimes say a lot about you just by saying his birthday? How credible is it?

According to Zhouyi, Eight Diagrams, Divination, Tarot, Constellation and other different categories. And some can use chess pieces and crystals, they all have certain principles and summarize different characteristics according to the operation characteristics of their respective methods. The general answer is very popular, that is, no matter what, there is always one that suits you, and some supernatural phenomena are really beyond our explanation, just like Bai Longwang, which is very popular, and many stars count and say yes. Some people are born with yin and yang eyes, and many things can hardly be explained clearly in words. If you believe it, it's the same, but if you don't believe it, you won't be forced to tell fortune, so it's harmless to do fun calculations. It can also be used as a reference, but I always believe that man can conquer nature. I recommend you to watch Superman by Miriam Yeung and Tony Leung Chiu Wai. Kangxi came to "Female Stars Love Fortune Telling" (it seems to be this name), and the ace spy also had a special fortune telling issue.