$(document).ready(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(200);
    }, function() {
            //Do the same again, only fadeOut this time.
            $(this).find('.sub_nav').fadeOut(80);
    });
    
});