I enjoyed this video. I was wondering how he was going to get a square from a circle, but all was revealed. I had my ahaaa moment, realising that you could create any shape from a circle of string, with each having internal angles add up to 360 degrees. The only exception I could immediately think of was that of triangles, although you could make the shape from a circle, the internal angles would only add up to 180 degrees. This would be a cool activity to do with kids.