private function init():void
    {
     stage.scaleMode = StageScaleMode.NO_SCALE;
     stage.align = StageAlign.TOP_LEFT;
     balls = new Array();
     for(var i:uint = 0; i < numBalls; i++)
     {
      var ball:animation_ch10_Ball = new animation_ch10_Ball();
      balls.push(ball);
      addChild(ball);
      ball.x = Math.random() * stage.stageWidth;
      ball.y = Math.random() * stage.stageHeight;
     }
     addEventListener(Event.ENTER_FRAME, onEnterFrame);
    }