CollapsiblePanelExtender

Question

I have an CollapsiblepanelExtender, only the Collapsecontrolid works ,not the Expandcontrol,Plealse Help.
Here is the sourcecode.
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>

<asp:CollapsiblePanelExtender ID="cpeEmployeeList" runat="server"
TargetControlID="panelContentEmployeeList" ExpandControlID="panelTitleEmployeeList" CollapseControlID="panelTitleEmployeeList"
Collapsed="true" ImageControlID="imgBackgroundEmployeeList" ExpandedImage="images/roundtable.jpg"
CollapsedImage="images/roundtable.jpg" SuppressPostBack="True" >
</asp:CollapsiblePanelExtender>
<asp:Panel ID="panelTitleIntroduction" runat="server" Height="25px" Width="20px">
<img id="imgBackgroundIntroduction" src="images/cal.gif" />
</asp:Panel>
<asp:Panel ID="panelContentIntroduction" runat="server" Height="160px" Width="415px">
<span style="font: 18px 'Lucida Grande', LucidaGrande, Verdana, sans-serif; color: #2d4c78;">The CollapsiblePanel is one of AJAX control in ASP.NET AJAX Control Toolkit. This tutorial combines CollapisblePanel and ASP.NET AJAX RC 1.0 to make amazing web application. It shows you how powerful in AJAX technology and what's advantage to use it. Hope you will like it</span>
</asp:Panel>



<asp:Panel ID="panelTitleEmployeeList" runat="server" Height="25px" Width="405px">
<img id="imgBackgroundEmployeeList" src="images/roundtable.jpg" />
</asp:Panel>
 
<asp:Panel ID="panelContentEmployeeList" runat="server" Height="310px" Width="415px">
<asp:Label ID="Label1" runat="server" Text="show me"></asp:Label>

<asp:GridView ID="GridView1" runat="server">
</asp:GridView>
</asp:Panel>