The source code of this page:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>javascript - Is stackoverflow using jQuery? - Stack Overflow</title>
<link rel="shortcut icon" href="http://cdn.sstatic.net/stackoverflow/img/favicon.ico">
<link rel="apple-touch-icon" href="http://cdn.sstatic.net/stackoverflow/img/apple-touch-icon.png">
<link rel="search" type="application/opensearchdescription+xml" title="Stack Overflow" href="https://stackoverflow.com/opensearch.xml">
<script type="text/javascript"
src="http://ajax.googleapis.com/ajax/libs/jquery/1.5.2/jquery.min.js"></script>
See that last line? As for sample code, look here.