This report describes the implementation of a laboratory exercise for an advanced biochemistry or enzyme kinetics class at the undergraduate or graduate level, designed to improve understanding of protein conformational changes associated with the binding of a ligand. Students measure the fluorescence changes induced by the conformational transition of a glycoprotein (the Na,K-ATPase) upon addition of different ligands (Pi and BeF[subscript 3][superscript -]) and analyse the results in order to determine the mechanism of the process. The results show that Pi and BeF[subscript 3][superscript -] present opposite effects on the observed rate constants (k[subscript obs]) with ligand concentration: k[subscript obs] decreases with [Pi] and increases with [BeF[subscript 3][superscript -]]. This observation, together with the frequently used assumption that binding occurs under rapid equilibrium, led to propose different models for ligand-induced conformational transitions: a "conformational selection" for Pi and an "induced fit" for BeF[subscript 3][superscript -]. In this paper, we show that if the rapid-equilibrium approximation for ligand binding is not assumed, a "conformational selection" mechanism can account for the effects of both ligands. This active-learning exercise serves as the basis for discussing the consequences of not being extremely cautious when invoking approximations about not-very-well-known systems and the importance of a correct understanding of models assigned to chemical processes.