Assign class through JQuery by selecting random ID
<div id="1">contents</div>
<div id="2">contents</div>
<div id="3">contents</div>
<div id="4">contents</div>
<div id="5">contents</div>
<div id="6">contents</div>

setInterval ( "flipit()", 3000 );

function flipit ( ) {
    $(document).ready(function(){
        var elmId = 'nS_' + (Math.floor(Math.random() * 6) + 1);
        $('#' + elmId).addClass('numSen2');
    });
}

var elmId = 'somePrefix_' + (Math.floor(Math.random() * 5) + 1);
$('#' + elmId).addClass('someClass');

<div class="addClasstoMe">contents</div>
<div class="addClasstoMe">contents</div>
<div class="addClasstoMe">contents</div>
<div class="addClasstoMe">contents</div>
<div class="addClasstoMe">contents</div>
<div class="addClasstoMe">contents</div>

var elements = $('.addClassToMe');
$(elements[Math.floor(Math.random()*elements.length)]).addClass('someClass');

$('div#'+~~(Math.random()*101)).hide();

var $divs = $('div'), len = $divs.length;
$divs.filter('#'+~~(Math.random()*++len)).hide();

var selector = '#' + (Math.floor(Math.random() * 50) + 1);

$(selector).html();

<div id="div-wrap">
    <div>contents</div>
    <div>contents</div>
    .
    .
    .
    <div>contents</div>
</div>

$(document).ready(function() {
    window.setInterval(function() {
        var $divs = $("#div-wrap").children("div");
        var randId = Math.floor(Math.random() * $divs.length);
        $divs.removeClass("current").eq(randId).addClass("current");
    }, 1000);
});​