While the three wise men were sleeping under a tree, the mischievous child painted their heads in red. Waking up, every sage discovered the child's handiwork on the heads of his friends. Naturally they began to laugh. Suddenly, one stopped talking. Why?

Answer: The sage stopped laughing because he realized that his head was also painted. This was preceded by the following thoughts: Let's say the heads are painted, only two other wise men, but then soon one of them will understand that his head is painted, because if it were not so, then the sage with the painted head would not have anything to laugh at. But the wise men do not stop laughing, so my assumption is wrong, and my head is also painted.

PS It is very funny that such a variant of the problem is usually solved quite quickly; If I replace three sages by one hundred, they too quickly guess that one must reason by induction and solve the problem, but if the number of sages is about seven, then people who solve this problem begin to "brake" and often even ask to answer ;-)