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Math Trek

Rolling with Reuleaux

9:55am, September 18, 2003

Have you ever wondered why the cover of a manhole is nearly always round? Why isn't it oval or square?

The usual answer is that a circular lid, unlike a square or an oval cover, won't fall through the opening. There's no way to position a round lid so that it can slip through a slightly smaller hole of the same shape. That's because a circle has a constant diameter. It is the same width all the way around.

In contrast, an oval or an ellipse is longer than it is wide. You can always find a way to slip an oval lid through a hole of the same shape. That's also true of a square or a hexagonal cover.

Amazingly, the circle isn't the only shape that wou

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