$(function () {
    // set opacity to nill on page load
    $("ul#menu span").css("opacity", "0");
    // on mouse over
    $("ul#menu span").hover(function () {
        // animate opacity to full
        $(this).stop().animate({
            opacity: 0.4
        }, 'slow');
    },
    // on mouse out
            function () {
                // animate opacity to nill
                $(this).stop().animate({
                    opacity: 0
                }, 'slow');
            });
});