In biological membranes that are permeable to water and ions but impermeable to other solutes, the diffusible ions cannot reach a concentration equilibrium. Instead, a state of electroneutrality is achieved on each side of the membrane, which requires that the diffusible ions be found in different concentrations on either side of the membrane. The Donnan equilibrium is a major contributing factor to the polarized state of cells, and appreciating it is vital to the understanding of neuronal physiology. This article presents a nonmathematical active-learning exercise that will help AP and college biology students understand how the Donnan equilibrium is achieved.