[jQuery] a problem with $.each() function

[Guapo]

when i write
$(document).ready(function(){
        $.each(document,function(k,v){
                document.writeln(k+"===="+v+"<br />);
        });
});
it works ,but when i change the document to window,as follows
$(document).ready(function(){
        $.each(window,function(k,v){
                document.writeln(k+"===="+v+"<br />);
        });
});
nothing output,why?
thank u all!