Thane Plambeck wrote:

I read that Dirac illustrated this fact, that a 360 degree rotation isn't the same as a 720 degree one, using strings tied to a pair of scissors, right?

My question: where/when did he do it?

The article The Spinor Spanner, Ethan D. Bolker, The American Mathematical Monthly, Vol. 80, No. 9. (Nov., 1973), pp. 977-984 suggests that this was a regular part of certain courses he taught. (Whether it answers the question asked or not, it's a lovely article. It's easy to build one -- 3 sufficiently long cords connected to a frame and to a spanner, wrench, scissors, piece of wood cut to the shape of a wrench, whatever -- and it's fun to play with.) John Robertson