Edit in JSFiddle

$('test').addEvent("click", function(e){
    e.target.getNext("span").set("text", "Hello World!").fade("in");
});

<div id="test">
   <p>
       <a href="#">Click me!</a><br />
       <span></span>
   </p>
</div>

#test{
    text-align:center;
}
#test a{
    font-size:12px;
}
#test span{
    font-size:22px;
    opacity:0;
}