I’ve had some friends lately go through the interview process (and some who will be soon). What I always try to impress upon people (and myself when I have to do it) is that the interviewer is bumbling around about as much, if not more than you are. I think the interview at my current job was pretty good, but then I remember this gem:

“Why is a manhole cover round?”

At the time I didn’t know that this was an open-ended question. I thought it was a riddle or a logic puzzle. I’m surprisingly good at these. I say surprisingly, but maybe it isn’t all that surprising when I answered:

“Because the hole is round.”

“…”

“Well, is that right?”

“Uh. There wasn’t supposed to be a right answer. It was just an open-ended question.”

“What else would someone even say?”

At the time I thought it was a dumb question, but another way to look at it is that they found out a lot by the way I answered. I work in a job that requires logical reasoning and troubleshooting all day long every day. We quest for the best and most logical answers. I thought I did well.

Still, I don’t know what other people answer this question with, so I thought I’d Google it (about two years after I was asked). Apparently the real reason manhole covers are round is because Microsoft started asking why in their interviews.

I was not the only one who gave the practical, boring answer, but some other answers:

“A round manhole cover cannot fall through its circular opening, whereas a square manhole cover may fall in if it were inserted diagonally in the hole. (A Reuleaux triangle or other curve of constant width would also serve this purpose, but round covers are much easier to manufacture. The existence of a “lip” holding up the lid means that the underlying hole is smaller than the cover, so that other shapes might suffice.)”

Okay, you got me. I’m not an engineer.

“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.”

I’m still not an engineer. This would explain why the internet is so strong, it also being constructed from a series of tubes.

“It’s easier to dig a circular hole and thus the cover is also circular.”

That’s a good point. I spent a lot of my youth in Cape Cod digging holes at the beach and I can’t say any of them were squares or triangles.

“The bearing surfaces of manhole frames and covers are machined to assure flatness and prevent them from becoming dislodged by traffic. Round castings are much easier to machine using a lathe.”

Yet again, these are things I didn’t realize. For all you machinists, score one for you.

“Human beings have a roughly circular cross-section.”

Here are the dumb answers I was looking for! Phew, all of these alternate answers were making me feel inadequate! This one I can totally feel superior to: “We’re kinda (but not really) circle, so they are too!”

“Tradition.”

…and I burst out into song, from Fiddler on the Roof. TRADITION! TRA-DI-TION! TRA-DI-TIOOOOON! To me, this answer is like saying: “Because that’s how God made it.” or just, “Because.” Even traditions have origins, people. I call FAIL on this answer.

Important to note, upon some investigation, not all manhole covers are actually round. Yes. The question itself makes an incorrect assumption. While round is most common, they also come in square and triangle. I think this easily ties into the message I was trying to send here. People who are interviewing currently, and will be soon as you graduate, don’t be too nervous as you interview. Know that the questions you will be asked and the people that ask them are flawed. They make incorrect assumptions, they steal things from Microsoft, and most have no right answer.

Just do the best to show your best, and you’ll do fine.

So, what do you answer if you get asked why is a manhole cover round?

Because Microsoft started asking people why.