Web Forms :: Can't Reference An Html Element From Code Behind In A User Control

Here is the contents of my test.ascx file:

<%@ Control Language="C#" AutoEventWireup="true" CodeFile="test.ascx.cs" Inherits="UserControls_test" %>
<p id="XXX">aaa</p>

and here is the contents of my test.ascx.cs file:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
public partial class UserControls_test : System.Web.UI.UserControl
{
protected void Page_Load(object sender, EventArgs e)
{
this.XXX.InnerHtml = "BBB";
}
}

I get an error when I refer to the id, "XXX" (underlined above) in code behind which reads: 'UserControls_test' does not contain a definition for 'XXX' and no extension method 'XXX' accepting a first argument of type 'UserControls_test' could be found (are you missing a using directive or an assembly reference?)

I have tried XXX.InnerHtml and I have tried this.XXX.InnerHtml and get same problem.

How do I successfully refer to ID'ed HTML elements in code behind?