private function onEnterFrame(event:Event):void
    {
     graphics.clear();
     for(var i:uint = 0; i < numParticles; i++)
     {
      var particle:animation_ch12_Ball = particles[i];
      particle.x += particle.vx;
      particle.y += particle.vy;
      if(particle.x > stage.stageWidth)
      {
       particle.x = 0;
      }
      else if(particle.x < 0)
      {
       particle.x = stage.stageWidth;
      }
      if(particle.y > stage.stageHeight)
      {
       particle.y = 0;
      }
      else if(particle.y < 0)
      {
       particle.y = stage.stageHeight;
      }
     }
       
     for(i=0; i < numParticles - 1; i++)
     {
      var partA:animation_ch12_Ball = particles[i];
      for(var j:uint = i + 1; j < numParticles; j++)
      {
       var partB:animation_ch12_Ball = particles[j];
       spring(partA, partB);
      }
     }
    }