为什么窨井盖是圆形的呢？
Why is the manhole cover round?
说起其中原因，还与数学有密切联系呢。
Speaking of the reasons, it is also closely related to mathematics.
第一，窨井设在大路上，每天走路的人来人往，设计时就要注意行人的安全，盖儿不能掉到窨井里。如果设计成三角形或者正方形的，盖儿虽然比窨井口大一些，但还是有掉下去的可能。而如果是圆形的，由于圆的直径相等，所以，盖儿只要大一点点，就不会掉下去。
First, the inspection well is located on the main road. The safety of pedestrians should be paid attention to when designing the inspection well for people who walk every day. The cover cannot fall into the inspection well. If it is designed as a triangle or a square, Gaier may fall down though it is larger than the inspection well head. And if it's round, because the diameter of the circle is the same, so as long as the cap is a little bigger, it won't fall down.
第二，由于窨井有时需要人工梳理或架线等，这时候又要求窨井的面积尽可能地大。在这些图形中，当它们的周长相等时，圆形的面积最大。同时圆形又符合我们的体型，便于工作人员进进出出。
Secondly, because inspection wells sometimes need to be combed or stringed manually, the area of inspection wells is required to be as large as possible. In these figures, when their perimeter is equal, the circle has the largest area. At the same time, the round shape is in line with our body shape, which is convenient for the staff to come in and out.
第三，三角形或正方形的边由直的线段组成，构成的角较尖，如果在修理时碰到很容易使人受伤。而圆形的边是一条圆弧，就不会出现这个问题。
Third, the edge of a triangle or square is composed of straight line segments with sharp corners, which can easily cause injury if it is encountered during repair. If the edge of a circle is an arc, this problem will not occur.