The Towers of Hanoi is a puzzle consisting of three posts    and several rings of various sizes.  The initial    configuration of the game is that all the rings are stacked    in decreasing order of size on one post.  The object of the    game is to move all the rings from one post to another    subject to two rules: only one ring can be moved at a time,    and at no time can one ring be placed on top of a smaller    ring.  The number of steps necessary to solve this puzzle is    exponential in the number rings.