<html><head><script src="https://libs.baidu.com/jquery/2.1.4/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>