I am having trouble conceptualizing how to have a containing box with a set size, and another box, much longer, that would animate from below it's containing box till its top meets the containers bottom edge.

I am having trouble figuring out how to approach this, mainly with the css, but I have a feeling too with the jQuery. I have used jQuery a bit, and am experienced with HTML and css, but I can't figure out how to do this. How would those elements be positioned, and would I use .animate()? Any help is appreciated.