activateMenu = function(nav) {

    var navroot = document.getElementById(nav);

    var lis = navroot.getElementsByTagName("LI");

    for (i=0 ; i < lis.length ; i++) {

            lis[i].onmouseover = function() {

                this.className += " over";

            }

            lis[i].onmouseout=function() {

                this.className = this.className.replace(" over", "");

            }


    }

}

window.onload= function() {

    activateMenu('mainNav');

}