<html>
<head>
<script src="https://libs.baidu.com/jquery/1.10.2/jquery.min.js">
</script>
<script>
$(document).ready(function(){
$("button").click(function(){
$("p").addClass(function(n){
return "par_" + n;
});
});
});
</script>
<style>
.par_0
{
color:blue;
}
.par_1
{
color:red;
}
</style>
</head>
<body>
<h1>This is a heading</h1>
<p>This is a paragraph.</p>
<p>This is another paragraph.</p>
<button>Add classes to p elements</button>
</body>
</html>