Simple example given below...
<div>
<asp:Panel runat="server" ID="Panel1">
<asp:Label runat="server" ID="lblMessage" Text="Remarks"></asp:Label>
          
<asp:Image ID="ImageArrow" runat="server" />
</asp:Panel>
<asp:Panel runat="server" ID="Panel2">
<asp:TextBox ID="TextBox1" runat="server" TextMode="MultiLine"></asp:TextBox>
<br />
</asp:Panel>
<ajax:CollapsiblePanelExtender ID="CollapsiblePanelExtender1" runat="server" AutoCollapse="false" AutoExpand="false"
CollapseControlID="Panel1" Collapsed="true" ExpandControlID="Panel1" TextLabelID="lblMessage"
CollapsedText="Show" ExpandedText="Hide" TargetControlID="Panel2"
ScrollContents="false" ExpandDirection="Vertical" ImageControlID="ImageArrow" CollapsedSize="0" ExpandedSize="120">
</ajax:CollapsiblePanelExtender>
</div>
for more detail check given link
http://www.asp.net/ajaxLibrary/AjaxControlToolkitSampleSite/CollapsiblePanel/CollapsiblePanel.aspx[
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