$(window).addEvent("domready", function() {
 $$("#topmenu .submenu").setStyles({
  opacity: 0
 });

 $$("#topmenu div.maindiv").addEvent("mouseenter", function() {
  this.getElement(".submenu").tween("opacity", 1);
 });
 $$("#topmenu div.maindiv").addEvent("mouseleave", function() {
  this.getElement(".submenu").tween("opacity", 0);
 });
 
});