What is fortune-telling psychological magic _ fortune-telling magic?

Guess how to play the magic of writing numbers in each other's heads.

Write a number on the paper that you think he can guess (small and light, usually a fortune teller writes 8). Hide something under your fingernails that can be written in black. Let him say the number he guessed. If it's the same, show it to him. If it's not the same, seize the opportunity to write down the figures he said quickly. Be careful not to look at the paper, but always look at him and talk to him.