This paper describes a short ion flow activity that can be completed within one lab or lecture session. The activity is focused on the core concept of flow-down gradients and is geared toward undergraduates. No previous knowledge of equilibrium potentials or membrane potentials is required. Students are guided through a set of questions that build in complexity. First, the K[superscript +] gradient across the membrane is considered. Simple questions are posed that allow students to build a foundation of basic facts regarding ion flow. Next, students work with the Nernst equation to investigate equilibrium potentials. In this part of the activity, students are presented with several sets of conditions in which the K[superscript +] gradient is made more or less steep, and they are asked to calculate the effect on the equilibrium potential. Students then write a description of the relationship between the magnitude of the gradient and the equilibrium potential. This is preparation for moving on to use a Nernst/Goldman simulator that allows manipulating the K[superscript +], Na[superscript +], and/or Cl[superscript -] gradients, as well as temperature and permeability. Finally, students answer questions that prompt them to summarize their knowledge regarding equilibrium potentials, membrane voltages, and the effect of temperature on the membrane voltage. By the end of the activity, students should be able to compare and contrast the Nernst and Goldman equations.