Here's an interesting logic puzzle (it is quite well known, see http://en.wikipedia.org/wiki/Common_knowledge_(logic) ). The version below is from Terence Tao's blog. xkcd also has a version ( http://xkcd.com/blue_eyes.html ). ***** – The logic puzzle – There is an island upon which a tribe resides. The tribe consists of 1000 people, with various eye colours. Yet, their religion forbids them to know their own eye color, or even to discuss the topic; thus, each resident can (and does) see the eye colors of all other residents, but has no way of discovering his or her own (there are no reflective surfaces). If a tribesperson does discover his or her own eye color, then their religion compels them to commit ritual suicide at noon the following day in the village square for all to witness. All the tribespeople are highly logical and devout, and they all know that each other is also highly logical and devout (and they all know that they all know that each other is highly logical a