JQuery bind()移除事件失效?


<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title>跟住鼠标移动的图片</title>    <script src="js/jquery-1.4.2.js" type="text/javascript"></script>
    <script type="text/javascript">
        $(function(){
            $(document).mousemove(function(e){
                $("#fly").css("left",e.pageX).css("top",e.pageY);
            })
            .mousedown(function(e){
                if(e.button == 1){
                    $(document).bind("mousemove"); 
                }
            });
        });
    </script>
</head>
<body>
<div id="fly" style="position:absolute"><img src="img/demo.gif"/></div>
</body>
</html>
当点击左键的时候将div固定在当前鼠标位置
用移除mousemove方法实现,但是无法实现,是什么问题?