为什么只有第一个li出来onmouseover效果？

       var Lis = document.getElementsByTagName("li");
            for(var i=0; i<Lis.length; i++){
                Lis[i].onmouseover = function(){
                    this.className = " lihover";
                }
                Lis[i].onmouseout = function(){
                    this.className = " ";
                }
            }