How do I automatically stack divs vertically inside a parent?

A div should already display as a block and take up a full “row”. Here is some HTML and CSS to give an example, compare it to your code:

http://jsfiddle.net/mWcWV/

<div id="parent">

    <div class="child">Foo</div>
    <div class="child">Bar</div>
    <div class="child">Baz</div>

</div>