$(window).addEvent("domready", function() {
 $$(".topmenu_item ul").setStyles ({
   "display": "block",
   "opacity": 0
 });

 $$(".topmenu_item").addEvent("mouseenter", function(event) {
  this.getElement("ul").tween("opacity", 1);
 });
 $$(".topmenu_item").addEvent("mouseleave", function(event) {
  this.getElement("ul").tween("opacity", 0);
 });
});