为什么big添加浮动后，small中margin-top是相对于big的，而不是body的

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>设置背景渐变</title>
<style type="text/css">
.big{
width: 300px;
height: 300px;
background-color: red;
margin-top:100px;
float: left;
}
.small{
width: 100px;
height: 100px;
background-color: blue;
margin-top:10px;
}
</style>
</head>
<body>
<div class="big">
<div class="small"></div>
</div>
</body>
</html>

Efficiency90