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?

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.