Edit in JSFiddle

$( document ).ready( function() {

  $( "#menu li" ).hover( 
    function() {
      $( this ).find( "ul" )
        .stop( true, true )
        .slideToggle();
    },
    function() {
      $( this ).find( "ul" )
        .stop( true, true )
        .slideUp();
    });
  
});