private function onEnterFrame(event:Event):void
    {
     var angle:Number = (mouseX - stage.stageWidth / 2) * .001;
     var cos:Number = Math.cos(angle);
     var sin:Number = Math.sin(angle);
     for(var i:uint = 0; i < numBalls; i++)
     {
      var ball:animation_ch10_Ball = balls[i];
      var x1:Number = ball.x - stage.stageWidth / 2;
      var y1:Number = ball.y - stage.stageHeight / 2;
      var x2:Number = cos * x1 - sin * y1;
      var y2:Number = cos * y1 + sin * x1;
      ball.x = stage.stageWidth / 2 + x2;
      ball.y = stage.stageHeight / 2 + y2;
     }
    }
   }
  }