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Why Are Manhole Covers Round? And Why It Matters

Jamie Hayes

Microsoft was once the largest tech company in the world, and for many aspirational computer engineers, a job there was the creme de la creme. But in order to get that job, the company had a seriously intense interview process. Applicants would have to sign non-disclosure agreements, interviews could take hours at a time, and the entire process could last months…but that wasn’t all. On top of all that, even if you’d prepared for weeks and considered every possible way the interview would go, the team at Microsoft might ask you something totally absurd; something completely unrelated to computer technology: “Why are manhole covers round?”


Manhole Covers What-Now?

Today, tech companies are famous for their bizarre, non-sequitur interview questions. Some other examples are “How do you measure four gallons of water using only a three and five-gallon jug?” and “How many window panes are there in the United States?” But one of the first of these is about manhole covers. The question has become so famous that it even appears in The Complete Idiot’s Guide to Job Interviews.

But why are these tech companies asking wannabe software engineers about manhole covers? What could that possibly have to do with the job they’re applying for? The answer is, Microsoft didn’t care if people knew why manhole covers were round. In fact, they were banking on people not knowing. The question is a test to see how a person thinks on their feet.

Joel Spolsky, a project manager from Microsoft back in those days, said the question revealed who was smart and could get stuff done, who was smart and couldn’t get stuff done, and who could get stuff done but wasn’t smart. That’s a lot of information for such an odd question.

A Complicated Question

The thing with “Why are manhole covers round?” is that there are tons of different ways to answer the question. There’s no right answer. Maybe it’s because round shapes are more efficient and cost-effective. Maybe it’s because we can make them more easily on a lathe. Round shapes are also easier for workers—they can move the covers around by rolling them, and they can dig a round hole much easier than they can make a square pit.

Fancier interviewees would maybe have said that round shapes are the best at resisting the compression of the Earth. Maybe they’d go so far as to mention how you can design a simple mechanism to lock a round manhole cover in place by turning it, something that would be much more difficult with a square.

Truly galaxy-brained applicants might point out that based on the geometry of a circle, a round manhole cover physically can’t fall down the hole it’s covering, while a square could. Or maybe some hopeful engineer would have simply shrugged their shoulders and replied, “I guess manhole covers are round because manholes are round,” and left it at that.

Answer Carefully

Microsoft didn’t ask people why manhole covers are round in the hopes of finding applicants with an encyclopedic knowledge of modern sewage infrastructure. They wanted to see who could think on their feet and approach a problem that they hadn’t prepared for. I’m guessing the people who came up with a quick, logical answer did better than the ones who took 15 minutes to come up with something inspired.

So if you’re ever in a job interview and someone throws you an absolute curveball, whether it’s this question or something else, don’t panic because you don’t know the right answer. Knowing it doesn’t hurt, but that’s not the point. The second you’ve panicked, you’ve already gotten it wrong.

Sources: 12.3, 4


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