Why this easy jquery code won't change the font-size
<p>Here's my jquery code. If you can't see any flaw in it, please proceed to see JSfiddle here at <a href="http://jsfiddle.net/J4ZgB/3/" rel="nofollow">http://jsfiddle.net/J4ZgB/3/</a> Thanks.</p>

<pre><code>$(function(){
    $("#a").click(function(){
        var s=$('#p_content').css("font-size");
        s+=4;
        $('#p_content').css("font-size",s+"px");
    })
})
</code></pre>
