Students will use the first law of thermodynamics to determine the feasibility of using corn ethanol as an alternative to fossil fuels in automobiles. Energy flow is tracked from the Sun, to photosynthesized carbohydrate, to ethanol through fermentation, and finally to work in the combustion engine. Feasibility is gauged by estimating a "corn-area-per-car" ratio: the area of Earth on which sufficient corn must grow to support the energy needs of an average motor vehicle. This exercise requires students to gain an understanding of the thermochemistry of combustion, fermentation, and the energetics of photosynthesis through an issue of current environmental significance.