这个js为什么实现不出来效果？
<script>
window.onload=function(){
var Lis=document.getElementsByTagName('li');
for(i=0;i<Lis.lenght;i++){
Lis[i].onmouseover = function(){
this.className="lihover";
}
Lis[i].onmouseout=function(){
this.className="";
}
}
}
</script>
.topmenu .lihover .submenu{
display:block;
}
