for(var i:uint = 0; i < numBalls; i++)
     {
      var ball:animation_ch15_Ball3D = new animation_ch15_Ball3D(3, Math.random() * 0xffffff);
      balls.push(ball);
      ball.ypos = -100;
      ball.vx = Math.random() * 6 - 3;
      ball.vy = Math.random() * 6 - 6;
      ball.vz = Math.random() * 6 - 3;
      addChild(ball);
     }
     addEventListener(Event.ENTER_FRAME, onEnterFrame);
    }