My answer would be because the hole that it cover is round as well. But here is a Microsoft job interview that further elaborates that. (Of course there are other answers of course.). But this one is fun.Round Manhole Covers, or: If Richard Feynman applied for a job at Microsoft
"Why are manhole covers round" is one of the eternal questions in
job interviews, and so it seems to be at Microsoft. The desired and
politically correct answer to the question is: "Manhole covers are
round because round is the only shape that can never fall into the
manhole and hurt someone (with the hole of the same shape, but slightly
smaller size than the cover)". And the answer is wrong.
Let's ask Mr Feynman:
"Interviewer: Now comes the part of the interview where we ask a
question to test your creative thinking ability. Don't think too hard
about it, just apply everyday common sense, and describe your reasoning
process. Here's the question: Why are manhole covers round?
Feynman: They're not. Some manhole covers are square. It's true
that there are SOME round ones, but I've seen square ones, and
rectangular ones.
Interviewer: But just considering the round ones, why are they round?
Feynman: If we are just considering the round ones, then they are round by definition. That statement is a tautology.
Interviewer: I mean, why are there round ones at all? Is there some particular value to having round ones?
Feynman: Yes. Round covers are used when the hole they are covering
up is also round. It's simplest to cover a round hole with a round
cover.
Interviewer: Can you think of a property of round covers that gives them an advantage over square ones?
Feynman: We have to look at what is under the cover to answer that
question. The hole below the cover is round because a cylinder is the
strongest shape against the compression of the earth around it. Also,
the term "manhole" implies a passage big enough for a man, and a human
being climbing down a ladder is roughly circular in cross-section. So a
cylindrical pipe is the natural shape for manholes. The covers are
simply the shape needed to cover up a cylinder.
Interviewer: Do you believe there is a safety issue? I mean, couldn't square covers fall into the hole and hurt someone?
Feynman: Not likely. Square covers are sometimes used on
prefabricated vaults where the access passage is also square. The cover
is larger than the passage, and sits on a ledge that supports it along
the entire perimeter. The covers are usually made of solid metal and
are very heavy. Let's assume a two-foot square opening and a ledge
width of 1-1/2 inches. In order to get it to fall in, you would have to
lift one side of the cover, then rotate it 30 degrees so that the cover
would clear the ledge, and then tilt the cover up nearly 45 degrees
from horizontal before the center of gravity would shift enough for it
to fall in. Yes, it's possible, but very unlikely. The people
authorized to open manhole covers could easily be trained to do it
safely. Applying common engineering sense, the shape of a manhole cover
is entirely determined by the shape of the opening it is intended to
cover.
Interviewer (troubled): Excuse me a moment; I have to discuss something with my management team. (Leaves room.)
(Interviewer returns after 10 minutes)
Interviewer: We are going to recommend you for immediate hiring into the marketing department."
:)
See Reuleaux Triangles
