Apply the CSS rule :nth-child(odd) {clear:both;}
See the live DEMO here
Cascading Style Sheets (CSS)
.container {border:solid #f00; padding:10px;overflow:auto}
.container div{border:solid 1px #f00; float:left; margin:5px}
.container div:nth-child(odd) {clear:both;}
HyperText Markup Language (HTML)
<div class="container">
<div>
Placeholder text for content<br>More content goes here.<br>Also, more content here.<br>And some more content.<br>Additional content.</div>
<div>
Another set of content individuals.
</div>
<div>
Some different collection of content items.
</div>
<div>
Miscellaneous content information.
</div>
<div>
Last set of random content.
</div>
</div>