Swimmy is an instructional tool for teaching neurophysiology that requires only a computer. Swimmy is a virtual fish that swims, moving its virtual tail by means of a virtual neural circuit.  Swimmy offers students the opportunity to approach a conceptually challenging task of scientific discovery and to get practical experience with the basics of neurophysiology--without the impediments of in vivo or in vitro neurophysiology experiments.  Swimmy is available for free, along with student and instructor tutorials.