点击桌面上的任意一个点，小球就可以沿着鼠标点击的方向运动，并实现碰撞到桌边的时候反弹，自动减速，自动变换方向。(Click on any point on the desktop, the ball can move along the direction of the mouse click, and to achieve the collision to the table when the rebound, automatic deceleration, automatic change direction.)