Linking two HTML SELECT's together



How do I link two HTML SELECT’s together. Let’s say for example that I
have a drop-down list with a list of all countries. Once the user
selects one of the countries I would like to make an AJAX call back to
the server to populate the second drop-down list with all of the cities
for that country.

Any ideas on how to go about to do this?

Jerrie Pelser