$(function() {
   //We initially hide the all dropdown menus
   $('#dropdown_nav li').find('.sub_nav').hide();
   //When hovering over the main nav link we find the dropdown menu to the corresponding link.
   $('#dropdown_nav li').hover(function() {
    //Find a child of 'this' with a class of .sub_nav and make the beauty fadeIn.
    $(this).find('.sub_nav').fadeIn(100);
   }, function() {
    //Do the same again, only fadeOut this time.
    $(this).find('.sub_nav').fadeOut(50);
   });
  });