To change the padding at the top of an element, we use the padding-top property.

<html>
<head>
<style>
.lightObjects {
padding-top:20px;
}
.heavyObjects {
padding-bottom: 20px;
padding-top: 60px;
}
p {
background-color: lightBlue;
}
</style>
</head>
<body>
<h3>Online Physics 101</h3>

<p class="lightObjects">Objects lighter than water float to the top
</p>
<p class="heavyObjects">Objects heavier than water sink to the bottom</p>
</body>
</html>]]>

Make sure to pick padding, followed by -, and finally top.