This question has an answer the interviewer *wants* to hear, and a *real* answer. The answers above come close to what the interviewer wants to hear, but the real answer is not nearly as pleasant... Microsoft should know better... the manhole covers at its Redmond site aren't even round! They are rectangular with curved corners. No trained individual would ever roll a manhole cover. They weigh a great deal and would pose a threat of injury if they were to fall on a hand or foot. Manhole covers are removed by use of a metal pole with a hook. They are slid or dragged where they need to go. In addition, there are numerous polygonal shapes that will not fall in on their own perimeter. And if you wanted to be absolutely certain a cover won't fall in a hole, you would just make every dimension of the cover larger than the largest dimension of the hole. (In fact, all covers today are larger than the holes they protect... even the circles.) The best , correct, answer as to why any given manhole cover is round is because the pipe which it caps is round. In fact, most manhole pipes have an inset lip forcing the cover to be round to fit within a circular projection. When you see curved rectangular covers, it is because the opening is a curved-corner rectangle. Again, this shape has a lip and a protrusion on the edges to hold it in place. Manhole covers are designed to fit the concrete, masonry, or metal cavities they are created to protect. This is the only true answer. Now, if the interviewer wanted to ask why the manhole pipes were round, that would be an entirely different question. I leave the research about Roman Aqueducts to the reader.

Nope. Covers are round because the hole is round. If the question was "why are manholes round", then one possible answer is "so the cover cannot fall through". There are other shapes that support this scenario BTW.

Easier to move round manhole covers around by rolling them. Also, easier to make a round hole than a square one