At Microsoft, job applicants are asked a number of questions designed to test their smarts.  At one point one question was “Why are manhole covers round?”  If you didn’t have an answer, you weren’t smart enough to work at Microsoft.  A rectangular cover can fall in, all you have to do is tilt it sideways.  Now you’re a step closer to getting that job at Microsoft.

Some other reasons include:

• Round tubes are the strongest and most material-efficient shape against the compression of the earth around them, and so it is natural that the cover of a round tube assume a circular shape.
• It is easier to dig a circular hole and thus the cover is also circular.
• A round manhole cover can be more easily moved by being rolled.
• If a manhole cover were to be displaced by a car or anything else, it could easily slip back into position and would not have to have any angles matched up.