The equilibrium between phases is a key concept from the introductory physical chemistry curriculum. Phase diagrams display which phase is the most stable at a given temperature and pressure. If more than one phase has the lowest Gibbs energy, then those phases are in equilibrium under those conditions. An activity designed to demonstrate the phase rule and to encourage deeper understanding of the Clapeyron equation is presented. This activity uses a mechanical drawing toy (such as an Etch A Sketch) with horizontal and vertical dials that can be adapted to explore mathematical relationships in a graphical situation. This activity is designed primarily for students taking second- or third-year physical chemistry, although some aspects of the activity would be appropriate for first-year general chemistry.