TED-Ed

View full lesson: http://ed.ted.com/lessons/why-are-manhole-covers-round-marc-chamberland Why are most manhole covers round? Sure it makes them easy to roll, and slide into place in any alignment. But there’s another, more compelling reason, involving a peculiar geometric property of circles and other shapes. Marc Chamberland explains curves of constant width and Barbier’s theorem. Lesson by Marc Chamberland, animation by Pew36 Animation Studios.