Does FormsAuthentication.SignOut() set Request.IsAuthenticated to false??

 Hi there,
I know what makes Request.IsAuthenticated true. But what makes it false?

I'm trying to set Request.IsAuthenticated to false by calling  FormsAuthentication.SignOut(), but withno luck.

In the markup file, I have a button Button1, and in the code-behind file I have the function which is fired when the button is clicked. The problem is the value of Request.IsAuthenticated remains true, so I never go to the default.aspx page.
Does anybody know  what makes Request.IsAuthenticated false.

Here is my code-behind file

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.Security;

public partial class mainpage : System.Web.UI.Page
{
    protected void Page_Loa ...