When we navigate through pages in a rails app, inturn we call one of the functions defined in the controller class. Lets say we access localhost:3000/articles/new then new action (method) of the ArticlesController class is called/invoked.It's simple.

But what i can't figure out is that since ArticlesController class is a pure Ruby class with some methods and we need an instance of this class to call one of it's methods. But we never explicitly do that. 
Then how the function call of any controllerclass is made possible ?