When you write a classic style Sinatra app, there's a class called `Sinatra::Application`.

When you call methods like `get` and `post`, it's modifying the Sinatra::Application class so that it can respond to HTTP requests passed to it by Rack.

Rack expects that whatever object it's being passed (in this case, the Sinatra::Application class) will have a method named `call` that accepts a single argument and returns an array.

See https://en.wikipedia.org/wiki/Rack_(web_server_interface) for details.

Zee