Why are manhole covers round?

Problem:  Why are manhole covers round?

Solution:  This is canonical with Microsoft, so it has to be asked.  Also, I was asked this at a Microsoft interview, so it's real!


There are lots of reasons that probably pop into your head, but let's not start by just randomly pushing out answers.  The place to start here is by examining, what is a manhole cover and what does it do?


Well, a manhole cover is a cover for an opening that provides a person access to an underground area (like a sewer or storm drain).  It is used so that people, objects, water and such do not fall in -- and so it can be removed when someone needs access.


So, a manhole cover would need to meet the requirements of:
1.  It is able to cover a manhole to prevent people/objects/water from entering
2.  It can be removed to provide access
3.  It is durable and strong as it may be outside and used aggressively (e.g., trucks may drive over it)
4.  It can be manufactured and made easily
5.  It should use the minimal amount of material needed to keep cost down
6.  It should take up the least space possible as it likely intrusive aesthetically and functionally (e.g., different surface than a road)


Well, let's now design the product that meets this. Clearly, to meet the first requirement, it must be  the same shape or bigger than the manhole.  And, since we want to keep meet requirements 4, 5, and 6, it makes no sense to make it any shape other than the cover itself.  So, manhole covers are round because they cover manholes which are round -- and this shape allows the cover to meet the requirements in the optimal manner.


There is a follow-up question here too -- why are most manholes round (thus creating their round covers).  Again, to answer this question, you would look at the requirements of a manhole:
1.  Allow a man to enter
2.  Cheap/easy to manufacture and maintain
3.  Of minimal required size as manhole creates constraints/weaknesses within their structure and are aesthetically ugly
4.  Durable/strong


Many shapes could meet some of the requirements, but round is often optimal as a round pipe is cheap/easy to manufacture and maximizes volume for a given amount of material.  Also, humans, with their arms in front as is often required to go down a latter -- fit well in a circular pipe.


So, manhole covers are round because it is the optimal shape to cover an optimally designed manhole.


The trick to this question is to realize that it is not a trick question, but a product design question at its core.  A wrong answer to this is starting to spout out true, but unorganized answers, (e.g., the manhole pipe is round,  it's a good shape, etc.).  Questions like this are designed primarily to see if a candidate is organized, hard to fluster and able to think in a structured manner.  


As an aside, not all manhole covers are round -- as sometimes the manhole cover is not round, usually due to constraints on the manhole.  Also, the round shape of a manhole contrasts it with the triangular shape of electrical covers, allowing people to know that a triangle means electricity, even if color or words have faded.


/* Please let me know if you find any bugs or alternative solutions.  Thank you, Noah */