Three wise men

Tired of disputes and summer heat, three ancient Greek philosophers lay down to rest a bit under the tree of the Academy garden and fell asleep. While they were sleeping, the jokers smeared coal on their foreheads. Waking up and looking at each other, everyone came in a cheerful mood and started laughing, but it didn’t disturb anyone, as it seemed to everyone that the other two were laughing at each other.



Suddenly one of the wise men stopped laughing, as he realized that his own forehead was also dirty.



How did he reason?

')

The task has long become a classic, if you know the answer, please be lenient with the topic.