Escher: which string do you pull from M.C. Escher's 1958 "Sphere Spirals" to escape?
non-Escher: graph theory - scroll to the middle of this page "Can you spot the difference between the graph above and the map to the left?" The PC's are shown a 3-dimensional image that flattens into the 2-D image (the "graph"). Later, the PC's enter a room with a floor colored like the "map." The space between the central G and Y is an illusionary floor leading to a pit trap.