This question is all about creativity and imagination, with a hint of knowledge. It’s one of the Microsoft interview questions that gained popularity in the 90’s. There is no definitive answer, but there are a number of reasonable ones.
Matter of fact:
“Because manholes are round.” This may lead to a follow up question, “why are manholes round.”
Practical matter:
“Because no matter how they’re positioned, round covers cannot fall through round manholes.” This isn’t actually true, as there is a whole class of polygons with constant width and diameter, but most interviewers won’t know this, even though the original interviewers at Microsoft did know this.
One of the reasons most cited by people who deal with manholes leads to this answer.
“It’s easier for a single person to move them around because, unlike any other shape, they can be rolled.”
Some answers take a little more imagination than others, but are still based in the real world.
“Round manhole covers have no edges. If one is dislodged and a car hits it, it won’t flatten the tires.” Still, people would probably not like the results of hitting a dislodged round manhole cover.
Some lesser used answers will lead to a more positive reaction from the interviewer:
Cost:
“It’s cheaper to manufacture a round manhole cover to fit a man sized round hole, than it is to manufacture a square man sized manhole cover that fits a square hole – there’s less surface area needed.” This answer has been mentioned by some engineers.
Practicality:
“When workers need to bring equipment, or cables, into the hole, they won’t snag on a round hole like they would on the edges of a square hole.”
The question has become so common that it often no longer serves its purpose. The original intent was not to get a specific answer, but to see how a person arrived at and explained whatever answer they gave. That included measuring people’s willingness to answer the question. The question was designed to measure logical thinking and open mindedness, more than knowledge of trivia. So if you can explain how you arrived at the answer, almost anything is fine.
Odd answer:
“Heavier people may have a harder time fitting into a manhole. They can more easily, and less painfully, squeeze into round holes than square holes or other shapes with corners.”
The question may seem silly, but avoid answering like this. “Nobody told me I’d have to crawl down manholes for an accountant job.”