window.addEvent('domready', function() { 
   var list = $('main_menu').firstChild;
   var menu = $(list).getChildren('li');
   menu.each(function(el){
      el.addEvents({
         'mouseenter': function(){
            this.getFirst('ul').setStyle('display','block');
         },
         'mouseleave': function(){
            this.getFirst('ul').setStyle('display','none');
         }
      }); 
      
   });
});