“为什么下水道的井盖都是圆的”，这可是微软面试的经典怪题。微软出这道题的目的如何，我们不得而知。不过，我能向您保证一件事，那就是看了下面这篇文章后，您肯定能从科学的角度回答对这个问题。

Manholes, which interconnect underground sewerage pipes, and serve as a point of entry for cleaning the pipes, are located at every major sewer pipe junction, and are capped with round manhole covers. The reason for the circular construction of these covers is, quite simply, that covers of any other shape would fall through the manholes by virtue of their varying diameters. Circular manhole covers do not vary in width, or in diameter, as is the case with these other shapes, thus remaining in place despite the street traffic running roughshod over them.

Manufacturers craft the manhole covers together with the smaller sized lip, upon which they rest, at the same time to ensure a tightly sealed fit. Additionally, the manufacture of circular manhole covers is easier and more accurate than the manufacture of covers of any other shape. Lastly, round manhole covers, once removed, require less lifting and less manpower, as their shape allows them to be rolled.

diameters: 直径

roughshod: 装有防滑钉的

（英语点津 Annabel 编辑）