This essay uses a physical modeling technique from mechanical engineering, the filling box, as a speculative architectural design tool. In the filling box, dyed salt water is injected into acrylic models submerged within a tank of fresh water, simulating the introduction of cold air into a warm environment or, when mirrored, the introduction of warm air in a cooler environment. The models make complex and beautiful convective thermodynamic processes visible, revealing insights about environmental processes taking place within and around buildings. Mirror images of model studies are accompanied by writing that draws on the science of thermodynamics to explore the atmospheric milieu of architecture, aligning an increasingly ubiquitous concept in architectural design discourse – thermal variability – with a design technique that foregrounds this concern.