[math-fun] Snaky puzzles

My column about snaky puzzles is up. http://www.maa.org/editorial/mathgames/mathgames_08_17_06.html I'm sure I missed a few things. I started looking at crossing numbers is graphs, for example. Consider a graph where every edge is crossed by N other edges. No edge crosses itself, or any edge it is adjacent to. Then that graph has writhing number N. All planar graphs have writhing number 0. A pentagon can have writhing number 2 (a thrackle). I wasn't able to figure out if a cubic graph could be anything but writhing-0, before I put my scribbles away and got back to finishing the column. --Ed Pegg Jr