Asp.net Ajex not working.

[thirith]

Hi all,

I am new in Asp.net Ajex. I am trying to use Ajex in my project but it doesnot working as I expected.

When i click on the button to change text of label, it refresh whole page not only label control.
my asp.net code:

asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
<div>
    <asp:UpdatePanel ID="UpdatePanel1" runat="server">
        <ContentTemplate>
            <asp:TextBox ID="txt" runat="server">
            </asp:TextBox>
            <asp:Label ID="lbl" runat="server">
            </asp:Label>
            <asp:Button ID="btn" runat="server" />
        </ContentTemplate>
    </asp:UpdatePanel>