

Why Are Manhole Covers Round? (on Microsoft brainteaser job interviews) - gruseom
http://www.hachettebookgroupusa.com/authorslounge/articles/2003/april/article16887.html

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a_caspis
Because they are easier to manufacture than other shapes which will not fall
through the hole either: <http://en.wikipedia.org/wiki/Reuleaux_triangle>

Want to attract nerd tourists ? Build your city with Reuleaux manhole covers
in the streets and Penrose tilings on the walkways.

~~~
ojbyrne
Seen in SF: <http://flickr.com/photos/ojbyrne/224171975/> :-)

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timcederman
So you can roll them out of the way more easily. Also - no orientation
required. My physics is kind of rusty, but isn't there a strength benefit from
the shape as well?

Incidentally, many sewer covers in Australia are square, only the larger ones
are round.

Okay, I'm done taking this article literally now. :)

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apathy
Good god, I'm dumber for having read that.

(Why is the cover the same shape as the hole? Because otherwise it's a pretty
shitty cover (i.e. gaps are bad))

It's astounding to me that people still care about these artifacts of another
era where people fawned over whichever corporate master they sought to please.
Good riddance to that, and if PG had never done anything else, he'd still be
able to point to that genius ad with "Larry and Sergey won't respect you in
the morning" and remain a hero.

~~~
gruseom
I couldn't agree more. I think it's an asinine practice that, if it is in fact
being discredited, deserves it. I posted this as an example of the brainteaser
interview being discussed elsewhere
(<http://news.ycombinator.com/item?id=162672>), and am a little disappointed
that most of the comments here are actually about manholes.

Brainteaser interviews are an artifact of the fundamental problem: there's no
easy way to identify good programmers, so people come up with these ersatz,
dysfunctional substitutes instead. The brainteaser is a seductive one because
it's easy to think "programmers are smart so let's test for smartness
instead". This confuses a necessary condition with a sufficient one. It also
begs the question, since smartness isn't easy to test for either. The concept
of "smartness" underlying the brainteaser interview is a shallow one of the
party-trick variety. I see no reason to believe that it's correlated with the
kind of programmer I'd want to work with on a project. Brain-teasing
opaqueness is not a good thing in production code, and real programming
problems are not much like riddles.

I also agree with your other point. Interviewers probing a programmer for
"smartness", like a horse might have its teeth examined, or better, like a cow
might have its udders palpitated - just feels unseemly to me. It's part of a
corporate culture that is the modern version of the feudal system. It's the
way you'd treat a peasant. The genius of the Larry-and-Sergey ad is that it
speaks truthfully about this issue of respect, which is a very deep one, and
of course is ultimately all about self-respect. (Look at some of the reactions
to PG's lion metaphor to see just how true that is.)

So large organizations have no reliable process for hiring good programmers.
As Paul Graham's writings have articulated so clearly, the fact that they have
that problem doesn't mean that you do; they need you more than you need them.
There's actually a much better test for which programmers add value in the
long run: the market itself. Pass that test and you won't have to ask anyone
to approve of your "smartness".

------
apgwoz
> "I'm happy to say that we're recommending you for immediate hiring into our
> marketing department."

It was worth reading this just for the Feynman anecdote at the end.

~~~
gruseom
Too bad the Feynman story is completely made up. It seemed pretty realistic.

I don't get the bit about the marketing department, though. How does cutting
through bullshit constitute an immediate qualification for marketing? The
story would make more sense if he were hired immediately for the technical
job. But that wouldn't be much of a punch line.

Edit: I was going to say that MS would never have interviewed Feynman, they
would have hired him immediately. But actually the anecdote is quite good in
this respect. Feynman would have insisted on going through the standard hiring
process.

------
diego
My guess would be that (many, most) manhole covers are round because it is the
obvious shape for most lids. There are so many poorly designed urban objects
that I tend to believe that the feature of not falling into the hole was a
lucky accident.

Think of stop signs, for example. It would be really easy to have a different
shape/color for four-way stops, but at least in the US all they do is add
"four way" to the sign in an inconsistent way.

~~~
LogicHoleFlaw
I think having multiple shapes for stop signs would be a _terrible_ idea. The
red octagon is an international standard. Messing with it is just asking for
trouble. The "4-way" signs are just an aid to drivers, to indicate what
behavior they can expect from other drivers. They're not required, and no
replacement for plain old good judgment.

------
tokipin
because manholes are round

~~~
Frocer
I think they are round because circular shape is the only shape that cannot
fall into itself.

If manholes are square or any other shape... at certain angles the cover could
always fall into the hole

~~~
hobbs
You can make a cover of any shape you want that won't fall in. It depends on
how much the cover overlaps the hole. Even with a round cover, you can't have
zero overlap or else the cover would fall in.

Now back to the correct answer, which is because the manholes are round - that
begs the question of why manholes are round. It's probably a matter of equally
distributing the sideways forces from the dirt plus a matter of being able to
manufacture round tubes more easily than, say, square tubes or hexagonal
tubes.

~~~
TrevorJ
Actually, no, a square cover would have to have a much wider flange in order
to ensure that it could not fall in . (rotated 45 degrees in relation to the
whole and tipped up, a square cover would fall in)

~~~
hobbs
Actually, yes, that's exactly what I was getting at. You'd have to make the
square cover sqrt(2) times wider than the hole, but you could still prevent it
from ever falling in. (Granted, it's not a very efficient solution, though.)

~~~
TrevorJ
Ok, agreed :-)

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mrtron
I didn't have a single brain teaser in my msft interviews!

I was almost disappointed because I am usually quite good at them.

~~~
gruseom
When was your interview? In another thread, somebody said that msft had mostly
given up on this practice.

~~~
mrtron
2006 I believe.

------
hopeless
Round ones are easier to screw on ;-)

Short answer: They're not. At least, they're not round everywhere in the
world. There are plenty of square and square-like covers in the UK/Ireland,
some of which split in half diagonally. Perhaps a better question is why
_should_ they be round or what advantages do round covers have?

~~~
Xichekolas
If they weren't round, superheros couldn't use them as frisbees/weapons.

If a car hits them and they bounce and spin, they will still (most likely)
settle back into the hole correctly. Even a Reuleaux polygon won't always do
that.

There are no points more highly stressed than others. Put an extreme amount of
weight on them and the force is evenly distributed around the circumference,
rather than overly stressing the corners and bending the cover (on a square
lid).

------
anamax
Manholes have flanges.

Circular and reuleaux triangles aren't the only shapes that won't fall
through.

------
mhb
So why are round shields better?

~~~
Xichekolas
Corners don't get caught on crap and slashes are deflected tangentially around
the circumference rather than cleaving the shield itself.

~~~
mhb
Thanks. Based on some shield images
([http://images.google.com/images?sourceid=mozclient&ie=UT...](http://images.google.com/images?sourceid=mozclient&ie=UTF-8&oe=utf-8&q=shield&um=1))
it looks like there are non-circular shapes which provide at least some of
those benefits as well as possibly improved coverage of an elongated object,
like a soldier.

