How would I change a drop down based on what is selected in a seperate drop down?

[Shpigford]

I've got 2 drop down select lists. The first one would have something like

-Category 1
-Category 2
-Category 3

Then what I'd like to happen is when you select one of those categories the next drop-down below it loads something like

--Sub Category 1
--Sub Category 2
--Sub Category 3

Any idea how I'd pull that off?

[stymiee]

You can find the code for that in this thread: http://www.sitepoint.com/forums/showthread.php?t=208541