how did he know

[JUANITAS]

"Here's the test: I have in front of me four hats. They're identical except that three of them are black, and one is white. I'm going to place one of these hats on each of your heads. There will be one hat left over. That hat will be removed from the room, as will all the mirrors. Before we put the hats on, you will be blindfolded. Then my assistants will help me to remove the blindfolds simultaneously, and you'll be seated so that you can see the other two participants. The first person to identify the color of his hat wins the job.

So they do it.

And as they're getting ready to remove the blindfolds, one guy raises his hand and says, "I know what color my hat is."

How did he know?

[hammersgirl]

Only a person seeing a white hat can give an answer, which is not fair to the person wearing it. So the white has been removed and only blacks remain.
That is why he can guess blindfold, because black is the only answer.

[BigMac]

For a variation, see this thread I started months ago....