The "Is my husband a cheat" riddle is actually very well known. It's just the Blue Eyed Islander puzzle in disguise: (http://xkcd.com/blue_eyes.html http://xkcd

The "Is my husband a cheat" riddle is actually very well known. It's just the Blue Eyed Islander puzzle in disguise: (http://xkcd.com/blue_eyes.html http://xkcd.com/blue_eyes.html) also known as the Muddy Children puzzle. Discussed here too: http://en.wikipedia.org/wiki/Common_knowledge_(logic) http://en.wikipedia.org/wiki/Common_knowledge_(logic) and here: http://terrytao.wordpress.com/2008/02/05/the-blue-eyed-islanders-puzzle/ http://terrytao.wordpress.com/2008/02/05/the-blue-eyed-islan... In fact, a whole book has been written about it and related issues: http://mitpress.mit.edu/catalog/item/default.asp?ttype=2&tid=8240 http://mitpress.mit.edu/catalog/item/default.asp?ttype=2&#38... It's a fascinating riddle because it introduces the subtle concept of "what somebody knows about what someone else knows about what someone else knows about X". But such a well discussed and complicated puzzle therefore also makes a bad, or at least "unfair", interview question. If you've heard it before, read that book, read Terence Tao's post on it... etc.. you'll easily ace this riddle. If you haven't heard of it before, it's going to take you way longer than just a few minutes to get your head around it. So as an interview question, it's testing what you "know", not how well you think.