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?
consider the that the prank was played on one sage only in that case we would have two sages laughing while the one on whom the prank was played would have been clueless to what was happening.
Similarly if the prank was played on 2 sages we would have had one sage lauging at two other sages. the two other sages wont laugh on the third if they see his forehead is clean.
Since all are laughing at each other one of the sage quickly realizes that the prank was played on all three of them.....
Solution
