changing the viewstate value

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

public partial class About : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        ViewState["xx"] = "x";
        if (!IsPostBack)
        {
            ViewState["xx"] = "y";
        }
        Response.Write(ViewState["xx"]);
    }
    protected void Button1_Click(object sender, EventArgs e)
    {

    }
}