If you need something to roll smoothly along the ground, you better strap some of those circular things called wheels to it, right? Well, not quite—because there are actually other shapes that roll smoothly, too.
See, if you rotate a square or hexagon or some other geometric shape along the ground, its top moves up and down with distance—making for a bumpy ride. Obviously circles don't have that problem: roll them, and their top stays at a constant height. But, as it turns out, shapes like the Reuleaux triangle or, um, British 50 pence coins, share the same quality too. Weird, huh? This video explains why the hell that works. [YouTube]