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Circular shape
The question of why manhole covers are typically round, at least in the 
U.S., was made famous by Microsoft when they began asking it as a 
job-interview question.[6][7][8] Originally meant as a psychological 
assessment of how one approaches a question with more than one correct 
answer, the problem has produced a number of alternate explanations, from 
the pragmatic ("Manhole covers are round because manholes are round.")[6] to 
the philosophical.
Reasons for the shape include:
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.)
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.
Similarly, it is easier to dig a circular hole and thus the cover is also 
circular.
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.
Circular covers do not need to be rotated to align them when covering a 
circular manhole.
Human beings have a roughly circular cross-section.
A round manhole cover can be more easily moved by being rolled.
If a cover had corners and were bent that would create a protruding point 
that could puncture tires.
Most manhole covers are made by a few large companies. A different shape 
would have to be custom made.
Other manhole shapes can be found, usually squares or rectangles. Nashua, 
New Hampshire may be unique in the U.S. for having triangular manhole covers 
that point in the direction of the underlying flow. The city is phasing out 
the triangles, which were made by a local foundry, because they are not 
large enough to meet modern safety standards, and larger triangles cannot be 
found.[9]
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http://en.wikipedia.org/wiki/Manhole_cover
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"Toxis" <toxis@nt_lietuvisko.tako> parašė naujienų 
news:ifsv9l$e5c$1@trimpas.omnitel.net...

A square manhole cover can be turned and dropped down the diagonal of the 
manhole. A round manhole cannot be dropped down the manhole. So for safety 
and practicality, all manhole covers should be round.