This riddle became famous as an interview question because it seems everyday, but it forces you to reason about geometry, safety, and practical use. The main answer is not in custom or aesthetics.
The round sewers
Riddle statement
Many manhole covers are circular. They could have other flat shapes—square, rectangular, triangular, or oval—but the round shape is repeated for some reason.
What is the decisive advantage of the circle in this case?
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Solution
Answer: A circular lid cannot fall into a circular hole of the same size.
The geometric reason is that the circle has the same width in any direction. It doesn't matter how you turn the lid: its diameter is always the same, so there is no orientation in which it can slip through the opening.
The same does not happen with other shapes. A square, for example, has a diagonal longer than its side; By tilting it, you can find a position where it fits through the gap. That is why the circle offers especially clean security.
In addition, it has practical advantages: it does not need to be placed in a specific orientation, it fits naturally with cylindrical accesses - common for structural reasons - and it can be rolled, something useful when the lid is heavy.