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?

You can also start with n=2, since in that case each philosopher, seeing that the other is laughing, would realize his own face was black.

Posted by Federico Kereki on 2004-04-13 10:51:23