private function init():void
    {
     balls = new Array();
     
     for(var i:uint = 0; i < numBalls; i++)
     {
      var ball:animation_ch09_Ball = new animation_ch09_Ball(Math.random() * 30 + 20, Math.random() * 0xffffff);
      ball.x = Math.random() * stage.stageWidth;
      ball.y = Math.random() * stage.stageHeight;
      ball.vx = Math.random() * 6 - 3;
      ball.vy = Math.random() * 6 - 3;
      addChild(ball);
      balls.push(ball);
     }