Così funziona

<html>
<head>
<script src="http://code.jquery.com/jquery-latest.js"></script>
<script type="text/javascript">
$(document).ready(function(){
$("p").hover(function(){
$("span").html( "SECONDN TEXT
SECOND TEXT" );
},function(){
$("span").html( "FIRST TEXT
FIRST TEXT" );
});
});
</script>
</head>
<body>
<p style="background-color:red; position:absolute"><span id="a">FIRST TEXT
FIRST TEXT</span></p>
</body>
</html>

Ciao