Is SelectedIndexChanged Event Working..?

Hi all,
I am new to .net, i developed simple page as
Default.aspx

<form id="form1" runat="server">
    <div>
        <asp:DropDownList ID="DropDownList1" runat="server"
            onselectedindexchanged="DropDownList1_SelectedIndexChanged">
            <asp:ListItem>1</asp:ListItem>
            <asp:ListItem>2</asp:ListItem>
        </asp:DropDownList>       
        <br />
        <asp:Label ID="Label1" runat="server" Text=" ...