It's AumSum Time
Why are manhole covers round?
Because they copied my shape.
No
Manhole covers are quite heavy.
So, round-shaped covers can be rolled easily from place to place.
Also, round covers can simply slide and fit into the hole.
But, to fit square or triangular manhole covers properly.
They must be precisely aligned with the edges and angles of the manhole.
Now, the manhole has a small lip around the rim of the hole.
This lip makes the opening smaller than its cover and prevents the cover from falling into the hole.
But, a square or rectangular cover can still fall into the hole, if inserted diagonally.
Why?
Because their sides are always smaller than their diagonals.
A round cover won't fall into the hole.
Because no matter how you place it, the cover is always wider than the opening.