To know what kind of file to include, the opening link tag needs the rel attribute set using rel="stylesheet".

<html>
<head>
</head>
<body>
<h1>Water Puns</h1>

<p>Why are rivers amazing roommates? They go with the flow.
</p>
</body>
</html>
]]>

Make sure to pick rel, followed by =, and finally "stylesheet".