RollOver function!?

Hey there…
I’ve got a tiny problem with a little rollOver effect that I’m trying to accomplish.

It’s probably pretty simple, but I can’t quite figure it out.
Here the thing:

I’ve got a button and a movieclip (20 frame animation), what I’m trying to do is, when you rollOver the button the animation starts to play, and it continues to play as long as the mouse is over the button. And then when you rollOut, the animation keeps playing untill it’s done with the last frame, and then stops.

How is this done…!?