Why My Whole Page Refreshes While Using ValidatorCallOut in Ajax Control Toolkit?

I downloaded Ajax Control Toolkit and want to add its components to my project. I Used ValidatorCallOut Component as an extender of a Textbox and when user changes focus to another Textbox ValidatorCallOut pops up. but this action will cause the page to refresh. How can I prevent that?

Here is my code:

<%@ Page Language="C#" MasterPageFile="~/User/UserMasterPage.master" AutoEventWireup="true"
    CodeFile="New.aspx.cs" Inherits="User_Accounts_New" Title="Untitled Page" %>

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

<asp:Content ID="Content2" ContentPlaceHolderID="LeftContentPlaceHolder" runat="Server">
    <act:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server" />

            <asp:TextBox ID="LoginUrlTxtb" CssClass="textEntry" TabIndex="0" runat="server" Width="321px"
                OnTextChanged="LoginUrlTxtb_TextChanged" AutoPostBack="True"></asp:TextBox>
            <asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server" 
                ErrorMessage="Url is Not Well Formated" ControlToValidate="LoginUrlTxtb">*</asp:RegularExpressionValidator>
            <act:ValidatorCalloutExtender ID="RegularExpressionValidator1_ValidatorCalloutExtender"
                runat="server" HighlightCssClass="validatorCalloutHighlight" 
                Enabled="True" TargetControlID="RegularExpressionValidator1" 
                CssClass="validatorCalloutHighlight" PopupPosition="Left" Width="200px">
            </act:ValidatorCalloutExtender>

            <asp:TextBox ID="FirstnameTxtb" CssClass="textEntry" TabIndex="开发者_JAVA百科0" runat="server" Width="321px"
                OnTextChanged="LoginUrlTxtb_TextChanged" AutoPostBack="True"></asp:TextBox>
</asp:Content>

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Your textboxes have the "AutoPostBack" property set to true. Therefore, whenever they lose focus, they will send a postback to the server and the page will refresh.