App Inventor's notional machine might surprise readers familiar with Scratch [8]. Scratch is also an event-based blocks programming language, but has a multi-threaded model in which multiple events can be executing at the same time. In the Scratch program in Fig. 3, when the green flag is clicked, a meowing sound is played repeatedly. If the sprite image on the screen is then clicked, the sprite moves 5 steps every tenth of a second while the meow sound continues to play. The executions of the play sound and move commands are interleaved, even though they are in different event bodies