You can know a person's last name by measuring your fingers. What is the principle?

This must be a scam. My guess is as follows:

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When most people hear that he can know his surname by measuring his fingers, they will unconsciously take a look at his surname on the cloth. At this time, the fortune teller can judge which area on the cloth is through your eyes, and then test it in front of you with a card. When the fortune teller moves his hand to a card, he will look you in the eye. Most people are a little nervous when they move their hands to their surnames, so measure their fingers.