At first glance you might assume the rules of Sudoku might come into play here, but they don't. Solving the six sides of this puzzle cube instead requires you to group the numbers into well-known mathematical patterns or collections—including a Fibonacci series, a prime number series, and even the digits of Pi. In other words, unless you've memorized these numberical formations, solving this puzzle will be all but impossible for anyone other than practising mathematicians. But it's great news for anyone with a math professor on their holiday shopping list, and a $20 limit. More here.