Q) Why is it that manhole covers are round?
The question achieved notoriety when it started being used by microsoft as an interview question. The answer that I’ve always liked is that a circular manhole cover can never fall into a circular hole. If you were to use a square, for example, it could easily fall through the diagonal.
So here’s another puzzle: is there any other shape for which this property holds – that you can make a manhole cover out of it that will never fall into the manhole?