Set the margin-top property to 25px.

<html>
<head>
<style>
.post {
margin-top: 25px;
margin-bottom: 25px;
padding-bottom:25px;
border: solid 2px lightBlue;
}
</style>
</head>
<body>
<h1>The Social Book</h1>
<h3>Notifications</h3>
<p class="post">Daniel checked in at Dublin Airport</p>
<p class="post">It's Sarah's birthday! Help her celebrate.</p>
</body>
</html>]]>

Make sure to type margin-top: 25px;.