One of Microsoft’s biggest contributions to humanity is the popularization of manhole covers. The most famous question that Microsoft asks during job interviews of geeks is probably, “Why are manhole covers round?” Supposedly the right answer is that if a manhole cover is round it can’t be dropped into the hole. See, for example, How Would You Move Mount Fuji? Microsoft’s Cult of the Puzzle - How the World’s Smartest Company Selects the Most Creative Thinkers. This book by William Poundstone is dedicated exclusively to Microsoft’s interview puzzles.
All we need, actually, is for the cover not to fit into the hole. For example, if the hole is small, the cover could be almost any shape, as long as the diameter of the cover is bigger than any straight segment that fits into the hole.
Microsoft makes an implicit assumption that the cover is about the same shape and size as the hole; otherwise we would waste a lot of extra cover material.
Even with this assumption, there is a good deal of flexibility in possible shapes if our only concern is that the cover shouldn’t fit into the hole. It is sufficient for the cover to be any shape with a constant width.
Here are some additional answers to why the cover should be round. Microsoft accepts the answer that a round cover is easier to roll. I’m not sure why a cover would ever have to be moved away from its hole. But I agree that if kids try to steal a cover, it would be much easier to escape with a round one.
Another answer that Microsoft supposedly accepts is that you do not need to rotate a round cover to align it with the opening when you are putting it back. This way if there is a lane divider painted on the cover it will point in a new random direction.
You can find many other explanations at the wiki article devoted to this subject. The most reasonable is that manhole covers are round because manholes are round. Duh!
Thanks to Microsoft there are now many websites with pictures of and discussions on the shape of manhole covers. For example, Manhole Covers Etc. or Manhole.ca or Manhole Covers of the World. As you can see many manhole covers are square or rectangular. They say that New Hampshire had triangular covers at some point.
But my favorite answer to this interview question was sent to me by Jorge Tierno:
Since manhole covers are not necessarily round, but you are asking why they are round, you are probably asking why round manhole covers are round. Round manhole covers are round by definition.
Now I have my own favorite question to ask you during a job interview: “Why are some manhole covers square?”