Diameter
直徑
A diameter is a straight line inside a circle.
直徑是圓內的直線。
Can any straight line in a circle be a diameter?
圓內的任何直線都是直徑嗎?
No, its two ends must meet at two different points on the circle.
不是,直線的兩端必須連在圓內的不同點上。
And the half point of the diameter should be the center of the circle.
而且直徑的中心是圓心。

Why do we need to know about diameters?
為什么我們需要了解直徑?
Because they are very useful when you draw a circle.
因為你畫圓的時候,直徑就非常有用。
Draw a circle that has a diameter of five centermeters.
畫一個直徑為五厘米的圓。
And ask your friend to draw a circle that has the same diameter.
并請你的朋友也畫一個直徑相同的圓。
Then compare the two circles.
然后對比一下這兩個圓。
Don't they look exactly the same?
它倆是不是一模一樣啊?