Relations between TVs

Hi guys !
I have this problem:
I have to edit a page (let's say I have a template of a city ) & I have a few properties:
- continent
- country
which are template variables (dropdown list menu) first with : Europe, Asia, Africa, America, Australia
now the interesting part comes... the second TV is also a dropdown list menu (for each continent a list of countries). My idea is when I change first TV (continents) the next TV should change too(a list of countries in this continents) similar to the effect of changing Template (you change the template... all the TV are changing too)
how can I do that , I've stumbled upon ... and nothing
can anyone please help me

thanks

Andrew

I think you would have an easier time doing that with a form in the front end and some JavaScript.