you can also roll the top.
you need not turn the top to fit it back.

to prevent a square from getting in, you need a hinge, which is extra cost.

Strictly speaking the round cover must be tapered, otherwise it CAN fall down the hole. This also means that it has to be fittdback the way it came out.

Same idea that applies to tunnels/pipes being round too..instead of squares or oblongs..

A round can be tapered to create a smooth ground surface or it can sit on a slightly smaller ring inside the hole, neither will fall through the hole. A square cover can be turned vertically and diagonally and it will fall through the hole.

man like to see holes not rectangles or triangles

Hey elena, if you turn a square vertically it would be the same unless squares have changed sense last time I've seen one, I am currently trapped inside of o blue triangle originating from the tropics of detroit...So cold...
*sobs*

Think in three dimensions, Emo. Elena is not referring to simply rotating the square. Lift one edge of the square manhole so that it is perpendicular to the ground. Then, swing one coner out over he hole, until it's lined diagonally across the opening. It'll slip in easily.

I can't believe I just gave a step by step description of that.

Square manhole covers are rarely used because they can fall through the square frame opening in the street if they are lowered vertically diagonal to the opening. This creates a safety hazard for men working below them so only round manholes and covers are typically used.

1. because circles are nicer.
2. becuase you can roll circles.
3. because if a square fell on someone down the sewers, a corner would do more damage than a rounded edge.

Round covers cannot be dropped or fall down a manhole, unlike square ones.

Well the actual reason is:-
A Man hole has to hold a lot of weight wne a vheicle passes on the road and as a circular object distrubutes the weight evenly throughtout the object..it is less likley to break..thats why a round man hole..and its cover..the reason given above also fits fine..

Manholes are round, the covers have to be round too.... Duh!!!! How can a square cover fit in a round manhole?????

Man Holes are PIPES. ROUND Pipes. The covers are also ROUND.