为什么“主要”部分边界被“旁”部分卡住了?为什么“旁边”部分和“主要”部分边界之间没有喘息的空间?

aside {
float: left;
width: 25%;
padding: 10px;
}
main {
border: 3px solid gray;
padding: 10px;
margin-left: 25%;
}
<aside>
<h2>The standard Lorem Ipsum passage</h2>
<p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore...</p>
</aside>
<main>
<h2>What is Lorem Ipsum?</h2>
<p>Lorem Ipsum is simply dummy text of the printing and typesetting industry...</p>
</main>