This is a generalisation of "The three sages
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?
(In reply to Is there something funny about a black face ?
I think there's a mistake there. B should rather think:
"A has a white face, and so do I (I hope). C surely also thinks so... then how come C isn't surprised at seeing D laugh? What would he be laughing at? As A's face is white, the only possibility is that my own face is also black!"