Back up your applications

Scenario
Let us suppose that you are using a very special application. It may be possible that after a long time of use, the repository from where you downloaded your application is not available any more. It may also be possible that, after a long use, something happened to your device, and you need to reinstall your favorite application.

I am going to explain you a way to keep a copy of the DEB package of your favorite application, so that you can install it again even if the package is not available any more from the repositories.