Modular controllers and indexes.

When you are modularizing your controllers how do you create a default
page for that module. For instance say you are creating an admin
section so you have your controllers in an admin folder, how do you
create the "index" controller. Put another way: if you had a folder:
app/controllers/admin/ with controllers such as
something_controller.rb defined in it's file as
Admin::SomethingController, what do you do so you can browse to
yoursite/admin and see a default controller such as
admin_controller.rb? Thanks!