问为什么不能使用mootools更改cookie值？

EN

<?php 
    setcookie('drres','hello');
?>
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/mootools/1.4.5/mootools-yui-compressed.js"></script>
<script type="text/javascript">

    function drres_cookie_read(){
        alert(Cookie.read('drres'));
    }
    function drres_cookie_write(){
        Cookie.write('drres','world');
        alert(Cookie.read('drres'));    // result is "hello" not "world",why?
    }
</script>
<button onclick="drres_cookie_read()">read</button>
<button onclick="drres_cookie_write()">write</button>