So you've mastered Rubik's Cube, eh. You're solving that thing in less than 30 seconds? Alright, try this one on for size. Rubik's 4 x 4 (aka Rubik's Revenge) increases the difficulty in solving the puzzle exponentially. Instead of having 6 sides of 9 cubes, you are faced with 6 sides of 16 cubes. Now let's see how fast you are...