I am attempting to use the CollapsiblePanelExtender but for some reason it never expands or collapses.. Do I need some codebehind for this tool to work?

Code:
<%@ Page Language="C#" MasterPageFile="MasterPage.Master" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="Default2" Title="Untitled Page" %>

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="atk" %>





<asp:Content ID="cbody" runat="server" ContentPlaceHolderID="body">

        <style type="text/css">
        .cpBody
        {
            background-color: #DCE4F9;
            font: normal 11px auto Verdana, Arial;
            border: 1px gray;                
            width:450px;
            height: 0px;
            padding: 4px;
            padding-top: 7px;
            overflow:hidden;
        }       
    </style>
            <asp:ScriptManager ID="ScriptManager1" runat="server">
    </asp:ScriptManager>
    <asp:UpdatePanel ID="upLR" runat="server" >
        <ContentTemplate>
        <atk:CollapsiblePanelExtender
    ID="cpe"
    Runat="server"     
    TargetControlID="pBody"
    CollapsedSize="0"
    ExpandedSize="300"
    Collapsed="True"
    ExpandControlID="pHeader"
    CollapseControlID="pHeader"
    AutoCollapse="False"
    AutoExpand="False"
    ScrollContents="True"
    TextLabelID="lblText"
    CollapsedText="Show Details..."
    ExpandedText="Hide Details" 
    ExpandDirection="Vertical" />
                    <asp:Panel ID="pHeader" runat="server">
                <asp:Label ID="lblText" runat="server" Text="License Request" />
            </asp:Panel>
 
            <asp:Panel ID="pBody" runat="server" CssClass="cpBody">
                    <ul>
                        <li><a href="">Production</a></li>
                        <li><a href="">Pre-Production</a></li>
                        <li><a href="">Development</a></li>
                    </ul>
            </asp:Panel>
 


        </ContentTemplate>
    </asp:UpdatePanel>

</asp:Content>