The Electronic Etch-a-Sketch™ uses two rotary potentiometers as inputs, and a Processing sketch as an output. The Arduino coninunously sends an X and a Y coordinate over the Serial interface. The Processing sketch uses these coordinates to move a draw pointer around a window, tracing as it does so, just like an Etch-a-Sketch™. You can use the serial monitor to observe the X and Y coordinates as they are sent.