function getMenus(){
  var menus = document.getElementById('top_nav');
  var sub = menus.getElementsByTagName('LI');
    for(i=0; i<sub.length; i++){
    sub[i].onmouseover = function(){
    this.className = "over";
    }
    sub[i].onmouseout = function(){
    this.className = "";
    }
  }
}window.onload=getMenus;