为什么验证码会不一样

[zackkey]

 
 private void GetImg()            //获取验证码图片
        {
            string url = "http://ptlogin2.qq.com/getimage?aid=1000108";
            HttpWebRequest myRequest = (HttpWebRequest)WebRequest.Create(url);
            
            myRequest.CookieContainer = CC;  //将COOKIE保存在CC
        
            HttpWebResponse myResponse = (HttpWebResponse)myRequest.GetResponse();
            Stream vfimg = myResponse.GetResponseStream();
            pictureBox1.Image = Image.FromStream(vfimg);

 //关联COOKIE再次读取验证码
            HttpWebRequest myRequest2 = (HttpWebRequest)WebRequest.Create(url);
            myRequest2.CookieContainer = CC;  //将COOKIE保存在CC
            myRequest2.Headers.Add(myRequest.Headers.Keys[0]);
            HttpWebResponse myResponse2 = (HttpWebResponse)myRequest2.GetResponse();

            Stream vfimg2 = myResponse2.GetResponseStream();
            pictureBox2.Image = Image.FromStream(vfimg2);

        }

为什么把我把上次的COOKIE关联了, 然后去再次读取验证码却不一样