Form Drop Down Menu that Open Dynamically

TecTaoMC used Ask the Experts™
I'm not sure if it is called dynamic drop down menus of not, but i am looking for code or a link to a tutorial where I can understand how to create what i would call dynamic drop down menus.

In a form there is a drop down menu, when one item is selected, an next drop down menu for sub categories is created.  I only need it for a main category and one sub category but I assume it could be done for many sub categories.
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Top Expert 2009
I would call them "dependent dropdowns". Here are two tutorials:

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