private function init():void
    {
     stage.scaleMode = StageScaleMode.NO_SCALE;
     stage.align = StageAlign.TOP_LEFT;
     ball = new animation_ch10_Ball();
     addChild(ball);
     ball.x = Math.random() * stage.stageWidth;
     ball.y = Math.random() * stage.stageHeight;
     addEventListener(Event.ENTER_FRAME, onEnterFrame);
    }