A diameter is a straight line inside a circle. 直径是圆内的直线。

　　Can any straight line in a circle be a diameter? 圆内的任何直线都是直径吗？

　　No, its two ends must meet at two different points on the circle. 不是，直线的两端必须连在圆内的不同点上。

　　And the half point of the diameter should be the center of the circle. 而且直径的中心是圆心

　　Why do we need to know about diameters? 为什么我们需要了解直径？

　　Because they are very useful when you draw a circle. 因为你画圆的时候，直径就非常有用。

　　Draw a circle that has a diameter of five centermeters. 画一个直径为五厘米的圆。

　　And ask your friend to draw a circle that has the same diameter. 并请你的朋友也画一个直径相同的圆。

　　Then compare the two circles. 然后对比一下这两个圆。

　　Don't they look exactly the same? 它俩是不是一模一样啊？