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);
      }
     }
    }