How to create a Sub Menu?

I know it can be done. But just can't find how to do a submenu that shows when you roll the mouse on main menu, and hides if your mouse is anywhere else. I can imagine that I can create the sub-menu with a movie clip symbol containing each command or choice as a button. But waht I don't know it how to keep it hidden and to make it only shown when you point the main menu that should activate ths sub menu.

Thanks for your help