Confusion About TextChanged Event Handler

In a previous discussion I was told that there was no need to add AutoPostBack="true" to every TextBox definition if AutoEventWireup="true" was in the page header.  If this is indeed the case then how come the TextChanged event is not firing when I type something into either of the two textboxes and then tab or click out of the textbox:
  <form id="form1" runat="server">
    <asp:ScriptManager ID="ScriptManager1" runat="server" />
    <div>
      <asp:UpdatePanel ID="UpdatePanel1" runat="server">
        <ContentTemplate>
          <asp:Panel ID="panelFields" runat="server" Width="300px" Height="500px" BorderColor="Black" BorderStyle=&q ...