This is a puzzle that has many variations, I heard this first time in Class XI, through my then class teacher Ravi Gopinath. The current version is from the book “The Moscow Puzzles” by Boris A. Kordemsky (Edited by Martin Gardner). Puzzle # is 279.
Three ancient Greek philosophers took a nap under a tree. While they were asleep, a prankster smeared their faces with charcoal. On awakening, they began to laugh, each thinking the other two were laughing at each other. Suddenly, one man stopped laughing. How did he realize that his own face was also smeared?
As always, please send your answers directly to me at email@example.com. If you like the puzzle, please share it with others. If you have interesting puzzles to share, please send them to me at my e-mail given above.
(Please note that I will be traveling the next two weekends and hence will take some before I post the answer to this one)