So Just Why *Are* Manhole Covers Round?

"Why are manhole covers round?" is a now famous interview question considered anywhere from thought provoking to downright stupid by people on the receiving end.

Why do interviewers ask it? Well, it's so well known now that they shouldn't anymore but originally it was to see how interviewees responded. It's a way to get a sense for an interview candidate's ability to think on his or her feet outside of their area of expertise. The way the candidate approaches the problem, thinks of and presents potential solutions, defends them, or discards them are all relevant to the interviewer.

It's a simple question. There are simple answers. But the great not-so-secret about these types of questions that everyone seems to miss is that it's not about the answer! The interviewer cares much more about how you arrive a whatever answer you get than what that answer is. Are you thinking logically? Is there method or madness to the solutions you consider, accept, and discard? Are you even willing to entertain such an absurd question?

Oh. You still want the answer.

The most common answer is simply that a round manhole cover can't fall into the hole it covers. But I did tell you that there's more than one answer. Heavy, round manhole covers are easier for one person to move by standing it on end and rolling. Also, you don't have to "align" a manhole cover to it's hole. Any orientation will do.

My father, a mechanical engineer, claimed an answer I hadn't heard before: that round manhole covers are easier to manufacture. That's a nifty answer and it would have done great in the interview except he also claimed that was the only reason and all the others were wrong. Whoops.

And for the record, round is not the ONLY shape that can't fall into its hole. There's a class of such shapes referred to as "Reuleaux polygons". The common characteristic is that they share a constant width or diameter.

I interviewed hundreds of technical candidates in my years at Microsoft. If you're going to be interviewing for a job, 10 Quick Steps to Interviewing for Tech Jobs will give you some insights on what the interviewers are really looking for when they talk to you. If you're an interviewer, 10 Quick Steps to Interviewing the Tech Jobseeker will give you some tips to help you identify and hire that star candidate you're looking for. Both are available now at 10quicksteps.com.