error in reading session variable in cakephp


I am using Sessions in Cakephp . I am having 3 controllers .In the User controller while logged in , i have wrote the User id in the Session variable .

But when i try to read it in the other controller i am getting the value of the Session variable.WHy so???

Edit :

In my users controller

i am writing the User id of the person who have logged in as

   function login()


	if (!$this->User->validates())
    		$this->Flash('Please enter valid inputs','/main' );

                    $this->Session->setFlash('Login Successful');

		$this->flash('Username and password do not match.','/main');



And in my forms controller i am trying to read the session variable user id like..,

  class FormsController extends AppController 

var $name = 'Forms';
var $helpers=array('Html','Ajax','Javascript','Form');
var $components = array( 'RequestHandler','Autocomplete');
var $uses=array('Form','User','Attribute','Result','Invite','Share','Choice');

 function index() {}

     function design() 
        	$userId=$this->Session->read('userId');echo $userId;


But this User id is not displaying the session variable what i have wrote....

Why its happening..