当鼠标移出灰色区域时候,菜单项怎么隐藏起来

<!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> 
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> 
<title>无标题文档</title> 
<script language="javascript"> 
var $=function(id){ 
    return document.getElementById(id); 
} 
function hover(){ 
    if($('hover').style.display == 'none'){ 
        $('hover').style.display = '';     
    }else{ 
    $('hover').style.display = 'none'; 
    } 
} 
</ script> 
<style> 
*{margin:10px; padding:0} 
#nav{border:#ccc 1px solid; height:25px; width:198px; text-align:center; line-height:25px; margin-bottom:0} 
#hover{ 
position:absolute; 
height:100px; 
width:200px; 
margin-top:-2px; 
text-align:center; 
background-color:#ccc; 
} 
</style> 
</head> 
<body> 
<div id="nav" onmouseover="return hover();"  onmouseout="return hover();">鼠标移上来</div> 
<div id="hover" style="display:none;"  onmouseover="return hover();"  onmouseout="return hover();"> 
  <ul> 
    <li>菜单一</li> 
    <li> 菜单二</li> 
  </ul> 
</div> 
</body> 
</html>

[Ctrl+A 全选注:如需引入外部Js需刷新才能执行]

当鼠标移出灰色区域时候,菜单项怎么隐藏起来_javascript技巧