How many people are wearing black hats (3 black hats and 2 white hats?

Three young people said they were the smartest. Then an old man came up and took out three black hats and two white hats to close their eyes, the old man put one of the three black hats on his head and asked them to open their eyes. He was the smartest to say who could tell the color of his hat. Three people looked at each other. someone suddenly said I knew it. What kind of black hat do I wear?
How did he know that he was wearing a black hat?

Answer:
1. It cannot be two white hats. Otherwise, one person may immediately know that he is wearing a black hat, because there are only two white hats.
2. If you have a white hat and wear it on your head
The other two will surely see one black and one white, because there may not be two white hats, so they will immediately know that they must wear black hats.
But they all face each other.
3. You must have a black hat on your head.