This is simply a fantastic example of applied geometry. This little paper toy is extremely easy to build and is even more amazing in person than its video lets on. Watch the video, then go here to get more detailed instructions on how to make it. After you make it, explore it: can you get it to count in order? What if instead of numbers you use colors, letter, shapes, etc.? Then, after really exploring it, can you figure out the secret behind how it works? HAVE FUN!