A life-cycle assessment (LCA) is a tool used by engineers to make measurements of net energy, greenhouse gas production, water consumption, and other items of concern. This article describes an activity designed to walk students through the qualitative part of an LCA. It asks them to consider the life-cycle costs of ethanol production, in terms of both energy consumption and carbon dioxide emissions. In the process, they trace matter and energy through a complex fuel-production system. (Contains 5 figures and 2 online resources.)