Most of you might be aware that, in those days, the mouse used to come with a ball inside, and there were two rollers that actually guide the computer where the ball is moving, and, accordingly, where the mouse is moving.
你们都知道，在那个年代， 鼠标里面有个球， 还有两个滚轮， 所以，当鼠标移动时， 滚轮会告诉计算机，球滚动的方向。