Microsofts Manhole Cover Question

A standard interview question asked by Microsoft interviewers. The question is typically phrased as "Why are manhole covers round?"

The interviewer is looking for lateral thinking skills with this question, of which the possible (correct) answers include:

Round manhole covers cannot fall into round manholes.

That would go for triangular covers too. Actually, it wouldn't. With an equilateral triangle, the distance between a given vertex and the center of the opposite edge is shorter than the length of each side, so it could still fall through. Of course, with other shapes of triangles, they'd fall through much more easily. -- KevinMcConnell

Round manhole covers can easily be rolled along the ground by one person.

Good reason for wheels to be round, but many heavy semi-permanent things, like JerseyWalls, aren't round and still work fine.

Round manhole covers do not need to be rotated to a specific orientation to fit over the hole.

Um, yeah, so I guess square doors and windows are just a common mistake? Their orientation is fixed by their hinges and frames.

My personal favorite answer is "Because the manholes are round!"

Why is the sky blue? Because it reflects the sea! [But that's just wrong.]

If they were square, you couldn't play Frisbee with them!

Actually you could. No aerodynamics in round frisbees; there are plenty of boomerang shaped frisbees for example. The only reason frisbees are round is to imitate the "flying saucers" of 50s SF flicks.

No, frisbees are round because the original "prototypes" were round pie tins thrown upside-down by Yale students. They're called frisbees because Wham-O (the original manufacturers) renamed their flying disc toys after the pie company "Frisbie". (Yes, with an "i"; presumably the change in spelling was for legal reasons.)

Hm... Here are my answers. Are any of them correct?

I don't know. I haven't studied the history of underground construction, but I can take some guesses.
Guess 1: Historical reasons. Somebody did it that way, people bought them from the first company to make them, and everyone else who wanted to get into the market had to make round covers and round manholes in order to interoperate with the existing installed base.

Um, yeah, just like how we all still programming in Fortran and Lisp.

A good number of people do seem to still have QwertySyndrome and use WindowsOperatingSystems. Many people use descendants of FortranLanguage and AlgolLanguage. See InnovatorsDilemma for other examples.

Guess 2: If you consider the amount of space that a person (or other object you want to stick down a manhole) takes up, and want to be able to turn the person (or other object) around, a circle is the shape of the smallest hole you can make while satisfying the constraint.

Ladder + projection of person's extremities makes a D shaped hole.

{"T" if the person is well-endowed.}

Guess 3: Manholes are basically big pipes. Pipes are usually round. Manholes (and hence manhole covers) might have been made round in order to allow for the use of commodity piping. Why are pipes usually round? I would guess that has something to do with structural integrity or not having any corners for crud to get stuck in, or maybe that's just the easiest shape to manufacture them in (cut a rectangle and bend it).

Winner! Bill, hire that man!

3 has always seemed most likely to me. Pipes are round because they have the most efficient surface area to volume ratio, and because they are easy to manufacture and somewhat stronger. Why were barrels and cups and jars usually round? Those were all precursors to pipes and manholes.
Also then you can screw the covers on. Next up: NivensManholeCoverQuestion.
- Most modern sewage pipes (the horizontal ones) are more egg shaped (with the pointed side down) then round. This makes it so that water flows faster at the relative narrow bottom and so prevents blockage. It needs to be wider at the top to accommodate for the shoulders of the inspectors.

Cause round things... are boring. -- FrankZappa

How about because circles are more pleasing, more natural to humans, it's a new use for out first great invention, the wheel.

Check out http://www.sellsbrothers.com/fun/msiview/#Feynman

I think if I'm ever answered that, my reply will be "Why are you still asking the same old hoary questions?" It'll reverse the responsibility as far as I'm concerned; at this point, you need a good reason to still be asking this stupid question, since it's legendary. Without a good answer I'm going to be very unimpressed. I couldn't give a complete list of good and bad responses ("Just answer the damn question!" is probably a big bad one), but you can probably imagine.

I guess I always thought the answer was that manhole covers are round because manholes are round. Did anyone ever try to dig a square hole?

I've heard someone else claim the question was "how many manhole covers are there in the US?" Either they got it wrong, this page got it wrong or Microsoft has a thing for manhole questions.

If I got asked this question I'd probably start by pointing out that they aren't. Most manholes in this country are square.

Maybe many are here also, we just wouldn't notice them, at least not as "manhole covers". Thus, there's some observer bias at play perhaps. -t

Ooo ooo I got one!

"I'm a phone screener for a huge company that's too rich to delegate any real decision making to any of its managers. So here's an empty-headed question for you, the hopeful applicant:

"'If a company with 200 phone screeners needs to fill 100 positions, phone-interviewing 10 applicants a day, what are the odds you will get bounced?'"

--PhlIp

Next big trend: interview banks in Bangalore.

I am very straight with my clients. I tell them up front that the two things I absolutely cannot tolerate in a workplace environment are incompetence and bullshit. Any form of this question smells heavily of both, which I would say out loud to the face of my interviewer(s). After that I'd ask them if they really wanted an answer, because for me to provide one would fall under engineering research services, for which I charge money. I'd tell them that for $75/hr, minimum two hours, two hour weekly retainer, I'd find out. Then they could waste up to another 119 minutes of my time before the billing clock would start back up again. -- MartySchrader

Often the questions are to find people who "fit in" to that particular organization. It's more of a cultural filter than it is a skills test. Humans are social creatures for good or bad, and social issues have a lot of influence on hiring decisions. -top

I guess that may make some sense for hiring captives, but I haven't had a captive job in 17 years now and I prolly won't ever again. It still irks me to think that some Big Firm would spring this kind of pop-psychological nonsense on an unsuspecting pleeb. And they wonder why pissed-off employees come to work with a gym bag full of guns to kill off the bosses...

Extra points for telling the interviewer to shut their man-hole.

Hardee-har har!

See: NarrowStaffSelectionFactors