CSS: Basic layout question - keeping nested elements inside each other

Question

I keep finding that if I have nested divs inside each other, and one of the inner ones is floated, the outer one won't expand around it.

Example:

<div style='background-color:red; '>
    asdfasdf
    <div style='float:left; background-color:blue; width:400px; height:400px;'>
        asdfasdfasdfasdfasdfasdfasdf<br />
        asdfasdfasdfasdfasdfasdfasdf<br />
        asdfasdfasdfasdfasdfasdfasdf<br />
        asdfasdfasdfasdfasdfasdfasdf<br />
        asdfasdfasdfasdfasdfasdfasdf<br />
        asdfasdfasdfasdfasdfasdfasdf<br />
        asdfasdfasdfasdfasdfasdfasdf<br />
        asdfasdfasdfasdfasdfasdfasdf<br />
        asdfasdfasdfasdfasdfasdfasdf<br />
        asdfasdfasdfasdfasdfasdfasdf<br />
        asdfasdfasdfasdfasdfasdfasdf<br />
    </div>
    asdfasdf
</div>

What do I need to do to the outer div to make it cover the inner one? IE: Put it's border/background color all the way around it?

Also, is there a general principle I am bumping up against here? If so, what should I look up to get a solid understanding of what it is?

Thanks!

Edit

Hi All,

Thanks for the answers, semantically correct and no, and for the links.

Though I will end up using overflow in the final work, I will leave Ant P's answer as accepted, as it was the first one that really worked, and got me out of a short term jam, even though it offends semantic sensibilities.

As a long-time html hack trying to move to decent css layouts, I can certainly understand, and sympathize with, using semantically incorrect hack that gets the job done, though I am sure he will change that habit after this =o)

Answer 1

You can do it strictly with CSS using overflow:hidden

<div style='background-color:red;overflow:hidden;'>
...
</div>

Answer 2

If you are the type that likes explanations (rather than just "do this") here are some excellent articles that explain several methods:

Simple Clearing of Floats

How to Clear Floats Without Structural Markup

Clearing Floats