You might be interested in this page: http://en.wikipedia.org/wiki/Reuleaux_triangle http://en.wikipedia.org/wiki/Reuleaux_triangle There is in fact a class of

You might be interested in this page: http://en.wikipedia.org/wiki/Reuleaux_triangle http://en.wikipedia.org/wiki/Reuleaux_triangle There is in fact a class of shapes that have the property that they can't fall into the hole which they cover (presuming a lip). In addition to the above reason about it being the most efficient way to cover a hole, a round cover is also easy to move if heavy (you can roll it), very useful for something that likely has to be sturdy but also move-able.