The button has four frames Up, Over, Down and Hit, you can create a movie tween
or anything you like in the over state, so that when the cursor is over the
button the timeline goes to frame 2 (Over) and you MC will play. For example
the buttons on the top of this page are all the same button, its just the text
on top of it that is different, so you don't have to create more than one
button. As you can see the over state on the button has a simple Alpha (0>100)
tween, of the same image at the Up state, which looks brighter because the mc
or the image has been modified.