I always thought it was because if you have a manhole cover that has angles on it, it's possible to turn the cover on its edge 90 degrees to the ground, and rotate around the central axis of the hole, such that it is possible it will *fall through the hole*. But a round cover has a constant radius, so there is no way to drop it through.
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branchandroot
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Date: 2023-03-25 10:53 pm (UTC)