function keeps repeating?

function keeps repeating?

It is a beginners problem I'm sure but I'm making a menu and when hovered, the submenu appear. However, when moving the mouse the submenu keeps fading in and out a few times. Why? Can I stop the function from repeating itself over and over?
  1.     $('#block-menu-primary-links ul:first li.expanded').hover(
        function(){
            $(this).children('ul').fadeIn(300);
        }, function(){
            $(this).children('ul').fadeOut(300);   
        });




With html:
  1. <div id="block-menu-primary-links">
     <div class="content">
     <ul class="menu">
     <li class="expanded active-trail"><a href="over">Over</a>
            <ul class="menu">
     <li class="leaf first active-trail">
     <a href="wat" class="active">Wat</a></li>
     </ul>
     </li>
     </ul>
     </div>
     </div> <!-- /.block --