Force child div to take 100% of parent height

[I Am Pikachu]

I am designing a fluid layout (no fixed px, all in %)I have the HTML as;\[code\]<div class="parent"><div class="fl child"><a class="prev"></a></div></div>\[/code\]If I check in Firebug, the height of parent is calculated as 400px..But if I give child as height:100%, it does not take the entire height..How do I fix this?