"How do you arrive at classes?"

  I keep simplifying and simplifying my Java course:
  during the first 40 hours all programs have one single
  class declaration only!

  Now, a student asked me: ?How do you arrive at classes??,
  in the sense of: ?How do you get to know which classes
  should be declared in your code, especially when there
  is more than one class declaration in the program??.

  (It might have a slight subtext, that he expects ?real
  programs? to have multiple classes [which is not even wrong!].)

  His question made sense, since I have very rarely or never
  shown them programs with multiple class declarations!
  (I showed already some ?advanced? stuff like non-static
  method declarations, and simple JavaFX windows with
  method literals, but all in a single class.) For example:

public class Main extends javafx.application.Application
{ public void start( final javafx.stage.Stage window )
  { final javafx.scene.control.TextField text
    = new javafx.scene.control.TextField( "10" );
    text.setOnAction
    ( e ->
      { text.setText
        ( java.lang.String.valueOf
          ( 3 * java.lang.Integer.valueOf
            ( text.getText() ).intValue() )); });
    final javafx.scene.Scene scene
    = new javafx.scene.Scene( text );
    window.setScene( scene ); window.show(); }}

  This was /all/ explained, they know method interfaces,
  but not yet programs with multiple class declarations.

  I tried to answer the question ?How do you arrive at
  classes?? in the classroom, kind-a stuttering, because
  I was not prepared for this question. Now, I am curious:
  How would /you/ answer this question?

  (An answer like ?high cohesion, low coupling? might still
  be too abstract. Can you find a reasonable answer that a
  beginner can understand? They know non-static fields and
  non-static methods [both use and declaration], variables,
  common statements and expressions, instance creation
  expression, references, reference variable and parameters.
  So a good answer would be ?hands-on? and build directly on
  these terms. What of the OOA/OOD rules can be most easily
  comprehended by a beginner?)