Css：div粘贴到父容器的顶部

Question

我的父容器内的div应该从容器的顶部开始有20px的边距，而边缘顶部应用于父容器......

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#rules-panel {
	background-color:blue;
	width:80%;
	margin:0 auto;
	height:800px;
}

#rules-panel .box {
	margin:20px;
  margin-top:20px; /* why isn't the margin working */
	background-color:red;
}

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<div id="rules-panel">

	<div class="box">
		<p>THIS IS div</p>
		<p>THIS IS div</p>
		<p>THIS IS div</p>
		<p>THIS IS div</p>
		<p>THIS IS div</p>
	</div>
  
</div>

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