jquery脚本错误

<!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>
    <title>jQuery basic</title>
    <style type="text/css">
        .selected 
        {
            background-color:Yellow;
        }
    </style>
    <script src="js/jquery-1.6.js" type="text/javascript"></script>
</head>
<body>
    <h3>jQuery构造函数</h3>
    <ul>
        <li>jQuery(expression,context)</li>
        <li>jQuery(html)</li>
        <li>jQuery(elements)</li>
        <li>jQuery(fn)</li>
    </ul>
    <script type="text/javascript">
$('ul>li:first').addClass('selected');
$('ul').append($('<li>new item</li>'));
$('ul').append($('<li>new item</li>'));
$(document).ready(function(){
            $('li:third').css('color','red');
        });
    </script>
</body>
</html>为什么$('ul>li:first').addClass('selected');改成$('ul>li:second').addClass('selected');显示脚本错误呀？