$(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: 1
        }, 150);
    },
    // on mouse out
    function () {
        // animate opacity to nill
        $(this).stop().animate({
            opacity: 0
        }, 150);
    });
});