for(var i:uint = 0; i < numParticles; i++)
     {
      var particle:animation_ch12_Ball = new animation_ch12_Ball(3, 0xffffff);
      particle.x = Math.random() * stage.stageWidth;
      particle.y = Math.random() * stage.stageHeight;
      particle.vx = Math.random() * 6 - 3;
      particle.vy = Math.random() * 6 - 3;
      addChild(particle);
      particles.push(particle);
     }