On a hot summer day, n equally bright philosophers, tired from all that philosophising, were napping in an orchard. A prankster came by, and painted all of their faces black with charcoal.

When the philosophers woke up, they started laughing at the others... until they suddenly realised all of their faces must be black!

How did they come to that conclusion?

The obvious answer seems to be that n>3 because of the grammar used in the question ("laughing at the others" - the "s" implying more than 1 other), so maybe when they saw that everyone else's faces were black, and they were laughing at him and he was laughing at them, that his face was black too.