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Why are manhole covers round?

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Why are manhole covers round?

Give the main geometric reason. Then explain precisely why a square cover could fall through its own hole, and describe a non-circular shape that would also be safe.

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Compare the diameter of a circle with the diagonal of a square.

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The theory behind this question.

Reuleaux Shapes and Why Manhole Covers Are RoundChoosing Coordinates That Kill the ProblemGeometric Probability PuzzlesThe Rope Around the EarthSphere Packing and Wasted Space

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