<html>
<head>
<script src="https://libs.baidu.com/jquery/1.10.2/jquery.min.js">
</script>
<script>
$(document).ready(function(){
$("p:nth-child(3)").css("background-color","yellow");
});
</script>
</head>
<body>
<h1>This is a heading in body</h1>
<p>The first paragraph in body.</p>
<p>The second paragraph in body (and the 3rd child element in body).</p>
<div style="border:1px solid;">
<span>This is a span element in div</span>
<p>The first paragraph in div.</p>
<p>The second paragraph in another div (and the 3rd child element in this div).</p>
<p>The last paragraph in div.</p>
</div><br>
<div style="border:1px solid;">
<p>The first paragraph in another div.</p>
<p>The second paragraph in another div.</p>
<p>The last paragraph in another div (and the 3rd child element in this div).</p>
</div>
<p>The last paragraph in body.</p>
</body>
</html>