当鼠标滑过按钮时显示下拉菜单的特效是怎么做的？

给个例子
<style type="text/css">
body{
    background-color:white;
    font-size:12px;
    font-family:Arial, Helvetica, sans-serif;
    margin:0px;
    padding:0px;
    color:white;
}
ul,li{
    margin:0px;
    padding:0px;
}
li{
    display:inline;
    list-style:none;
    list-style-position:outside;
    text-align:center;
    font-weight:bold;
    float:left;
}
a:link{
    color:#336601;
    text-decoration:none;
    float:left;
    width:100px;
    padding:3px 5px 0px 5px;
}
a:visited{
    color:#336601;
    text-decoration:none;
    float:left;
    padding:3px 5px 0px 5px;
    width:100px;
}
a:hover{
    color:white;
    float:left;
    padding:3px 3px 0px 20px;
    width:88px;
    text-decoration:none;
    background-color:#539D26;
}
a:active{
    color:white;
    float:left;
    padding:3px 3px 0px 20px;
    width:88px;
    text-decoration:none;
    background-color:#BD06B4;
}
#nav{
    width:600px;
    height:30px;
    border-bottom:0px;
    padding:0px 5px;
    position:absolute;
    z-index:1;
    left: 198px;
    top: 25px;
}
.list{
    line-height:20px;
    text-align:left;
    padding:4px;
    font-weight:normal;
}
.menu1{
    width:120px;
    height:auto;
    margin:6px 4px 0px 0px;
    border:1px solid #9CDD75;
    background-color:#F1FBEC;
    color:#336601;
    padding:6px 0px 0px 0px;
    cursor:hand;
    overflow-y:hidden;
    filter:Alpha(opacity=70);
    -moz-opacity:0.7;
}
.menu2{
    width:120px;
    height:18px;
    margin:6px 4px 0px 0px;
    background-color:#F5F5F5;
    color:#999999;
    border:1px solid #EEE8DD;
    padding:6px 0px 0px 0px;
    overflow-y:hidden;
    cursor:hand;
}
</style>
<div id="nav">
    <ul>
    <li class="menu2" onMouseOver="this.className='menu1'" onMouseOut="this.className='menu2'">我的首页   
    <div class="list">
        <a href="#">CSS</a><br />
        <a href="#">我的首页</a><br />
        <a href="#">我的日志</a><br />
        <a href="#">我的日志</a><br />
        <a href="#">我的相册</a><br />
        <a href="#">我的收藏</a><br />
    </div>
    </li>
    <li class="menu2" onMouseOver="this.className='menu1'" onMouseOut="this.className='menu2'">社区圈子
    <div class="list">
        <a href="#">CSS</a><br />
        <a href="#">我的首页</a><br />
        <a href="#">我的日志</a><br />
        <a href="#">我的相册</a><br />
        <a href="#">我的收藏</a><br />
    </div>
    </li>
    <li class="menu2" onMouseOver="this.className='menu1'" onMouseOut="this.className='menu2'">我的短信   
    <div class="list">
        <a href="#">CSS</a><br />
        <a href="#">我的相册</a><br />
        <a href="#">我的收藏</a><br />
    </div>
    </li>
    <li class="menu2" onMouseOver="this.className='menu1'" onMouseOut="this.className='menu2'">账户管理   
    <div class="list">
        <a href="#">CSS</a><br />
        <a href="#">我的首页</a><br />
        <a href="#">我的日志</a><br />
        <a href="#">我的相册</a><br />
        <a href="#">我的收藏</a><br />
        <a href="#">我的日志</a><br />
        <a href="#">我的相册</a><br />
        <a href="#">我的收藏</a><br />
    </div>
    </li>
    </ul>
</div>

看了，收获很大，thanks