One field's contents reacting to the previous field

[Luvvie]

I'm setting up our Collectibles database, which is a pretty basic catalogue system. I'm nearly finished but really struggling with what should be a relatively simple task, but the last time I used FM Pro was version 4 with no training, and I can't spend any more time trying to fathom this!

I have pre-existing categories and sub-categories which pertain to them... I've set the CATEGORY field as a drop-down value list and I'm trying to make it so that, when I've selected (eg) BOOKS from the value list, the SUB CATEGORY field will automatically show the BOOKS Sub Categories value list...

Does that make sense? I'm now utterly confused as to whether this should be a script, a calculation, a relationship, or what!

If anyone can help me I'd be immensely grateful, as I'm now just about at the hair-pulling stage.

Yours hopefully, Jules.

[comment]

See if this helps:

http://www.fmforums.com/forum/showtopic.php?tid/193221/post/281976/#281976