private function onEnterFrame(event:Event):void
    {
     graphics.clear();
     graphics.lineStyle(1);
     graphics.moveTo(mouseX, mouseY);
     moveBall(balls[0], mouseX, mouseY);
     graphics.lineTo(balls[0].x, balls[0].y);
     
     for(var i:uint = 1; i < numBalls; i++)
     {
      var ballA:animation_ch08_Ball = balls[i-1];
      var ballB:animation_ch08_Ball = balls[i];
      moveBall(ballB, ballA.x, ballA.y);
      graphics.lineTo(ballB.x, ballB.y);
     }
    }