Transitions for movieclips... is it possible?

I basically want to add transitions to just movieclips or at least i think that would be the appropriate term. The problem is I want to have a sub menu that expands and contracts. So basically I want when lets say button “1” is pushed submenu, lets call “sub1” slides in. Then When you click “2”, “sub1” slides out and “sub2” slides in. I don’t really have a clue how this would work but any help is appreciated.

By the way, it will probably done in like 2 weeks but I have been working on my FIRST site ever. It was the reason I joined kirupa in my search for knowledge. Before this site, which I am still doing, I had not done a SINGLE Flash or PS ever. I mean ever, I had never even seen them let alone know that ActionScript even existed! I think you guys will like it since I am spending about 2 months on it (and Im still not done). I guess I am one of those people who don’t like to put out mediocre work when i know its mediore. Well Thanks for the help.