Here a numerical simulation model, implemented in Excel, that can be used to expand student understanding of the theory of customs unions is presented. The model allows examination of core customs union issues including trade creation, trade diversion, and the KempWan theorem. By responding to an experimental assignment that guides them through the simulation, students are able to learn the theory in a hands-on manner. We also attempt to measure the effectiveness of our approach, and find evidence to suggest exposure to simulation improves student outcomes.