The key is that you can examine the state of most (not all) "intermediate" variables so not only do you know the inputs, the outputs, but also everything that h

The key is that you can examine the state of most (not all) "intermediate" variables so not only do you know the inputs, the outputs, but also everything that happened in between in all these different ways. Your calendar app adds 2 days for tasks? You can just see the cell where it says 2 to make sure that variable is set right. Its amazing, and we are just catching up to it with "always on" variable inspectors in IDEs. Edit: That's not the only trick: cells don't have variable names but locations. You don't have to remember the type (they are all cells) or the name of a variable. Its just a location and you click on it to select it.