private function init():void
    {
     ball0 = new animation_ch08_Ball(20);
     addChild(ball0);
     ball1 = new animation_ch08_Ball(20);
     addChild(ball1);
     ball2 = new animation_ch08_Ball(20);
     addChild(ball2);
     addEventListener(Event.ENTER_FRAME, onEnterFrame);
    }