UpdatePanel does not work for me (full postback)... need help please

Please, somebody can help me?

My page have a full postback when i click the button....why?

My code is only this:

The 

ASPX:

<%@ Page Title="" Language="vb" AutoEventWireup="false" EnableEventValidation="false" MasterPageFile="~/PROWEB.Net/Site.Master" CodeBehind="CTR10190.aspx.vb" Inherits="ProjetoPROWEB.CTR10190" %><%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc2" %><asp:Content ID="Content1" ContentPlaceHolderID="MainContent" runat="server"><asp:ScriptManager ID="ScriptManager1" EnablePartialRendering="true" runat="server"></asp:ScriptManager><div id="teste"><asp:UpdatePanel ID="UpdatePanel3" runat="server"><ContentTemplate><br /><asp:Button ID="Button1" runat="server" Text="Button"   /><br /></ContentTemplate></asp:UpdatePanel></div></asp:Content>

Thanks for any help/suggestion!