Hardly the hardest. Here's a harder:<p><a href="http://justinpombrio.net/tell/prisoner-lightbulb.html" rel="nofollow">http://justinpombrio.net/tell/prisoner-lightbulb.html</a><p>EDIT: Also, for anyone stuck on the three gods puzzle, the three sisters puzzle is a good stepping stone:<p><a href="http://mathpuzzlewiki.com/index.php?title=Three_princesses" rel="nofollow">http://mathpuzzlewiki.com/index.php?title=Three_princesses</a>