Remove all the pieces from a chess board except for one knight. Then try to move the knight across all 64 squares of the board, touching each once. (As a reminder, knights move in a L-shape, two spaces in one direction, and then one space left or right, or up or down, at a 90- degree angle.) This so-called "knight's tour" is very difficult to achieve for a single person, but mathematicians have calculated that there are a mind-boggling number of ways to pull it off. If you end up at the spot you started, you'd be completing a so-called "closed tour." There are more than 26 trillion ways to do this. If you merely touch every spot, without returning to your point of origin, it's called an open tour. The number of ways to do this is so large that scientists haven't calculated it.