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