I suggest you go read up on the slding doors technique for making expanding buttons. It's well documented and used a lot.
A List Apart: Articles: Sliding Doors of CSS
google reveals several variations and some prebuilt menus eg
Free CSS Navigation Menu Designs at ExplodingBoy
Stu Nicholls | CSSplay | Experiments with cascading style sheets | Doing it with Style has some variations too.
Stu Nicholls | CSSplay | Fitted Doors

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