See css: http://jsfiddle.net/4JgA4/114/
Also here:
<div style="min-height:40px; border:solid 1px black; float:left;">
<div style="border:solid 1px black; height:150px; float:left;">
First to set height
</div>
<div style="border:solid 1px red; height:100%; float:left;">
Why don't get the 100%
</div>
</div>
Why does the second div doesn't get the 100%?