Demo:
Bret Victor - Inventing on Principle
Essay
From the essay:
Alan Perlis wrote, “To understand a program, you must become both the machine and the program.” This view is a mistake, and it is this widespread and virulent mistake that keeps programming a difficult and obscure art. A person is not a machine, and should not be forced to think like one.
Examples of the same idea:
http://www.pythontutor.com/
http://www.vizit.tech
Cheers and happy coding 