如何得知点击在指定区域外？

抠门，哈<!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> new document </title>
    <meta name="generator" content="editplus" />
    <meta name="author" content="Gao YiXiang" />
    <meta name="email" content="[email protected]" />
    <meta name="keywords" content="javascript dhtml dom" />
    <meta name="description" content="I love web development." />
    <style type="text/css">
html, body
{
    margin: 0px 0px 0px 0px;
    height: 100%;
}
    </style>
</head>
<body onclick="alert('out');">
    <div style="width: 100px; height: 100px; background-color: yellow;" onclick="alert('in');event.cancelBubble=true;">
        
    </div>
</body>
</html>