In this paper, we describe laboratory and classroom exercises designed to obtain the "fundamental" equation of a rubber band by combining experiments and theory. The procedure shows students how classical thermodynamics formalism can help to obtain empirical equations of state by constraining and guiding in the construction of the physical models for the system under investigation. It also serves as an "experimental" link to statistical thermodynamics models for a rubber band, which is taught to students in more advanced courses. The rubber band system also serves as an example of a "non-ideal" thermodynamics system to introduce a van der Waals-like equation in a context different from gases.