Problems with div not assuming total height

Problems with div not assuming total height

I am having some problems with this situation:
http://jsfiddle.net/gespinha/wgWAx/4/
The JQuery alert should report the #parentDiv height, and, as you can see,
its original height is 2000+ and not 532 as is displays.
This happens because its height is set to 100%, but how can I make this
100% mark the page height, or should I say, the original #parentDiv's
height?
<div id="parentDiv">
<header id="childDiv">
<div id="anotherDiv"></div>
</header>
<div id="spaceDiv"></div>
</div>