Human beings are having a profound impact on the environment. The opportunity to investigate this timely issue during one or two class periods gives algebra and precalculus students insight into a sustainability topic of great international concern--carbon footprints. Students use mathematical thinking in matters that are pertinent to their everyday lives. Further, they use key quantitative reasoning and mathematical modeling skills: interpreting units, computing with large values, using assumptions to derive an appropriate mathematical model (in this case, a linear model), using models to make predictions, and understanding when a model is appropriate for making predictions--practices aligned with Principles and Standards for School Mathematics (NCTM 2000) and the Common Core Standards (CCSSI 2010). This article presents a classroom-tested activity that uses authentic, diet-related data to focus on sustainability. A bibliography is included.