Unrelated, but following one of the footnotes led me to this interesting page on using color to visualize four-dimensional movement. It uses untying knots as an

Unrelated, but following one of the footnotes led me to this interesting page on using color to visualize four-dimensional movement. It uses untying knots as an example. It's a small thing, but it worked well for me and made the concept of a Klein bottle much more clear: http://www.rdrop.com/~half/Creations/Puzzles/visualizing.4D/index.html http://www.rdrop.com/~half/Creations/Puzzles/visualizing.4D/...