just to be amusingly pedantic: - The manhole cover should be thick metal - in some third world countries this is actually a problem because the heavy manhle co

just to be amusingly pedantic: - The manhole cover should be thick metal - in some third world countries this is actually a problem because the heavy manhle covers have a substantial value as scrap metal, and thus get stolen. The heavier they are, the more cash they will bring in. - circles are the only shape I know of that follow this - a reuleaux triangle can't fall into the hole either. Incidentally this is also the shape used for the turning part of a wankel engine. (http://en.wikipedia.org/wiki/Reuleaux_triangle http://en.wikipedia.org/wiki/Reuleaux_triangle) :-)