Changing a Variable Over Time

This is one of those things that I assume is either really easy or really hard to do. I have a variable that AS3 assigns some different values to at a couple points in a project (in this case to adjust volume). Normally when you assign a new value, say slider.x = 1, the adjustment is instantaneous. Is there anyway to make a new value assignment gradual? So if slider.x = 1, and the project calls a line of code that says slider.x = 100, I can have that transition over a few seconds rather than instantly? Either by telling it to slowly transition, or by some kind of trick like having it add 5 to slider.x per second for the next 20 seconds?
Thanks for any advice.

Wow; that was surprisingly easy to work with, given my limited experience with Flash to date. Thanks!
-Tom