HTML elements not properly stacking in Twitter's 'Bootstrap' framework

I'm curious why the following won't display properly:

<div class="topbar">...</div>
<d开发者_JS百科iv class="container">...</div>

I'm using Twitter's Bootstrap (http://twitter.github.com/bootstrap/) for the layout and the contents within topbar are on top of the contents within container. Why aren't these divs properly stacking on top of each other?

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After reading the documentation regarding the use of the <topbar> element, it reads:

Note: When using the topbar on any page, be sure to account for the overlap it causes by adding padding-top: 40px; to your body.