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!