Stolen from the html5 boilerplate

    <!-- Grab Google CDN's jQuery. fall back to local if necessary -->
    <script src="//ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.js"></script>
    <script>!window.jQuery && document.write(unescape('%3Cscript src="js/libs/jquery-1.4.2.js"%3E%3C/script%3E'))</script>