According to Joel Spolsky, one of Microsoft's early program managers, questions like these distinguish between people who are smart and get things done, people who are smart but don't get things done, and people who get things done but are not smart.

This question was made famous when Microsoft began using it as a job interview question. It is an example of a creative problem-solving question that is used to assess how a person approaches a problem with more than one possible answer, and to test their logic, common sense and ability to think through an unusual question.

Microsoft must have been onto something, because this type of question has been adopted by many other companies as an alternative to more traditional interview questions.

A number of possible explanations for why manhole covers are round include:

► It's easier to dig a circular hole

► Human beings have a roughly circular cross-section.

► Circular covers don't need to be rotated or precisely aligned when placing them on the opening.

► A round manhole cover is easily moved and rolled.

► Round tubes are the strongest shape against the compression of the earth around them, so the cover of the tube would naturally be round as well.

► A round manhole cover cannot fall through its circular opening, whereas a square manhole cover could fall in if it were inserted diagonally in the hole.

► Round castings are much easier to manufacture using a lathe.