We report a case study of model-based reasoning in which a small group of fourth-grade students analyzes the energy flow when a solar panel is used to power an electric motor that spins a propeller. In developing their explanation of energy flow, the students draw on a general model of energy developed collectively by their class in the course of an experimental classroom curriculum led by a trained teacher. They also construct a model-based representation of the specific system under study. Their investigation and reasoning process exhibit all the features of authentic scientific model-based inquiry, including the revision of their models to incorporate new information. In the course of their work the students recruit and seamlessly integrate nearly all of the practices of science designated in the Next Generation Science Standards. This case study provides an example of what modeling-based teaching and learning can look like in an elementary school classroom. It also suggests that the study of energy offers a particularly promising context for developing students' use of science practices, especially the practice of developing and using models.