On a hot summer day, tired by long discussions, three wise sages were slumbering inthe shade of a tree. A prankster was walking by, and couldn't pass up such an opportunity. He took a piece of charcoal, and put a black spot in the middle of each sage's forhead, careful not to wake them.

When the men did wake, they looked at each other, saw the black marks and began laughing. Each one thought that his two friends had the mark, while his own forhead was clean.

Suddenly, one of them stopped laughing, having realized that his forhead must be dirty as well. How did he arrive at this conclusion?