tweening question

I'm trying to tween a shape (rectangle). Basicaly, make an on mouse over event that makes the shape elongate. So if figured "easy", I'll just make two keyframes and tween between the two. Well, it sort of works but the shapes moves as it is tweening. I believe this is due to the fact that the pivot point is moving with it. So if could just move the pivot point to one end this wouldn't happen. I could be wrong, but this is what I think is going on. Let me try to draw what I want to do in good ol' ASCI art.

go from here:
_______________
|_______________|

(On mouse over) to here:
___________________________
|___________________________|

(On mouse out) back to here:
_______________
|_______________|

question.
is there any way to move the pivot point to one end so that the shape doesn't move as it elongates?
If there is a better anwer, please let me know.

BTW: I just been working with flash for about 2 days so I'm a noob in it.

i've used other animation software like maya, 3ds max and Toonz so don't worry about animation jargon.

thanks

(wow. I haven't use shape tween in ages, and had to lookup how to do it. *beet red*)

if you're using shape tween to elongate your button... on the first keyframe of the tween, click on your shape, and look in the Info panel. there are nine little squares. you can set your center point of your shape to center (by clicking the middle square) or top left (by clicking the top-left square).

or you can move the center pivot by clicking the Free Transform tool, and then drag the pivot to wherever you want.

thanks