Whoops, how'd this slip down so far??

Here's another one:
The Three Small BoysThree small boys were talking together when they were joined by an older man. The newcomer noticed that each of the threee boys had a smudge of dirt on his forehead.
"Boys," he said, "will each of you look at the foreheads of the other two and if you see a smudge of dirt on either or both, raise your hand." All three boys looked and all three raised their hands.
"Now," said the old gentleman, "if one of you is certain that he has dirt on his
own forehead and can tell me how and why he knows this, he is to raise his hand and I will give him a quarter."
The three boys looked at each other for a few moments, and one of them suddenly raised his hand. Can you figure out how he knew that he had a smudge on his forehead?