<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "​​http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd​​​">
<html xmlns="​​​http://www.w3.org/1999/xhtml​​​">
<head runat="server">
    <title></title>

    <script src="jquery-1.3.2.min.js" type="text/javascript"></script>

    <script type="text/javascript">
        $(document).ready(function() {
            $("#list").click(function() {
                $(this).css("background-color", "red");
                $($(this).siblings()).css("background-color", "blue");
            });
        });
    </script>

</head>
<body>
    <form id="form1" runat="server">
    <div id="list">
        aa</div>
    <div>
        bb</div>
    <div>
        cc</div>
    <div>
        dd</div>
    </form>
</body>
</html>