含有子元素的标签响应鼠标事件的问题

事件只绑定到div上，所以没反应
没有明白楼主要实现什么

就是想让整个DIV内容区响应onmouseover，移出DIV内容区响应onmouseout，现在是移到DIV内部的A、SPAN等的标签上都算移出了DIV，响应了onmouseout，其实根本没出DIV的范围
<!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=gb2312" />
</head> <body>
<div id="demo" style="background-color:#eee; border:1px solid #ccc;">
    <a href="#">This is a link </a>
    <span>This is a plain text </span>
    <img src="../../JS/practice/20070120114354739.jpg" width="200" height="150"/>
</div>
<script type="text/javascript" language="javascript">
document.getElementById("demo").onmouseover = function(mEvent){
var obj;
if (typeof(event) != "undefined"){ // 判断是否为IE浏览器
obj = event.srcElement;
}else{
obj = mEvent.target;
} var o = window.event.fromElement;
while( o ){
if( o == document.getElementById("demo")){
o = window.event.toElement;
while( o ){
if( o == document.getElementById("demo")){
return;
}
o = o.parentNode;
}
break;
}
o = o.parentNode;
}
alert("Mouse over");
}
</script>
</body>
</html>
syukugai的方法是我想要的结果，谢谢！给分结贴了！