$(document).ready(function(){

  var original_nav_html = $('#sub_nav').html();
  var hover_sub_nav = false;

  $('#main_nav').hover(function(){

    //hover over the sub nav
    hover_sub_nav = true;
  }, function() {

    //roll off the sub nav
    $('#sub_nav').html(original_nav_html);
  });

  $('.primary_nav li').hover(function() {

    //hover over the main nav
    if($(this).children('ul').html() != null){
      $('#sub_nav').html('<ul class="sub">'+$(this).children('ul').html()+'</ul>');
    }else{
      $('#sub_nav').html('<ul class="sub"></ul>');
    }

  }, function(){

  });

});