private function onEnterFrame(event:Event):void
    {
     for(var i:uint = 0; i < numBalls; i++)
     {
      var ball:animation_ch11_Ball = balls[i];
      ball.x += ball.vx;
      ball.y += ball.vy;
      checkWalls(ball);
     }
     for(i = 0; i < numBalls - 1; i++)
     {
      var ballA:animation_ch11_Ball = balls[i];
      for(var j:Number = i + 1; j < numBalls; j++)
      {
       var ballB:animation_ch11_Ball = balls[j];
       checkCollision(ballA, ballB);
      }
     }
    }