for(var j:uint = i + 1; j < numBalls; j++)
      {
       var ball1:animation_ch09_Ball = balls[j];
       var dx:Number = ball1.x - ball0.x;
       var dy:Number = ball1.y - ball0.y;
       var dist:Number = Math.sqrt(dx * dx + dy * dy);
       var minDist:Number = ball0.radius + ball1.radius;