Vi Hart's video on hexaflexagon was featured on io9 yesterday:
After seeing the hexaflexagon video I ended up visiting her YouTube channel, full of awesome videos re: math and geometric shapes. What really blew my mind was the Möbius strip music box. A Möbius strip is essentially a band that has only one surface: If you take a strip of paper and then tape it end to end, what you have is a ring with two surfaces (inside and outside). But, if you turn one of the ends before taping them, you end up with the Möbius strip that has only one surface - there is no inside or outside anymore! And using this, you can actually keep playing the notes on the Möbius strip in the music box without ever hitting the end.
Anyways, here are just two examples of how awesome her videos are. I am very impressed. Highly recommend that you subscribe to her channel.