Adding controls dynamically to an UpdatePanel in ASP.NET AJAX

I have the following really simple code

<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>

<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
    <asp:PlaceHolder ID="PlaceHolder1" runat="server">
    </asp:PlaceHolder>
    <asp:Button ID="Button1" runat="server" Text="Button" onclick="Button1_Click" />
</ContentTemplate>
</asp:UpdatePanel>

And the codebehind

protected void Button1_Click(object sender, EventArgs e)
{
    Literal literal = new Literal();
    literal.Text = DateTime.Now.ToString();
    literal.ID = DateTime.Now.Ticks.ToString();

    // These both work fine the first time the button is clicked
    // but the second time nothing is added.
    UpdatePanel1.ContentTemplateContainer.Controls.Add(literal);
    PlaceHolder1.Controls.Add(literal);
}

My problem comes in that the Literal control is only ever added once. I've scoured google and blog sites (plus books) but without any luck. What am I missing?