jquery 下拉菜单

<div class="wrap">
    <ul>
        <li>
            <a href="javascript:void(0);">一级菜单1</a>
            <ul class="ul">
                <li><a href="javascript:void(0);">二级菜单11</a></li>
                <li><a href="javascript:void(0);">二级菜单12</a></li>
                <li><a href="javascript:void(0);">二级菜单13</a></li>
            </ul>
        </li>
        <li>
            <a href="javascript:void(0);">一级菜单2</a>
            <ul>
                <li><a href="javascript:void(0);">二级菜单21</a></li>
                <li><a href="javascript:void(0);">二级菜单22</a></li>
                <li><a href="javascript:void(0);">二级菜单23</a></li>
            </ul>
        </li>
        <li>
            <a href="javascript:void(0);">一级菜单3</a>
            <ul>
                <li><a href="javascript:void(0);">二级菜单31</a></li>
                <li><a href="javascript:void(0);">二级菜单32</a></li>
                <li><a href="javascript:void(0);">二级菜单33</a></li>
            </ul>
        </li>
    </ul>
　　</div>

<script>
    $(function () {
       var $li=$(".wrap>ul>li");
       //给li注册鼠标经过事件，让自己的ul显示出来
        $li.mouseover(function () {
            $(this).children("ul").stop().slideDown();//jq 转换 DOM
        });
        $li.mouseout(function () {
           $(this).children("ul").stop().slideUp();
        });
    });
</script>