In App Inventor, all computation is initiated by event handlers associated with components. For example, Fig. 2 shows a program that increments the number in the Counter label when the CountButton is pressed and resets this label to 0 whenever Screen1 of the app is visited. The when CountButton.Click block is an event handler that executes the event body code (labelled do) whenever CountButton is pressed. The Initialize event handler for a screen is the only thing in App Inventor that resembles a main entry point to a program. However, it rarely does anything beyond initializing the state of the screen's program. The main work of the app is done by all the other event handlers associated with the screen. The app's "program" is its set of event handlers