Imagine saving just one of the 23,000 species threatened with extinction. Students studying endangered species in a general life science course faced the decision of which species to save as a summative assignment in a unit on scientific explanation and argumentation. They used the claim, evidence, and reasoning (CER) framework in which students first look at their data, then decide what claim is best supported by their evidence, and, finally, connect the evidence to the science, using reasoning to explain how the evidence supports their claim. Before this activity, students practiced CER and argumentation with other scenarios. For example, they analyzed a "crime scene" photo and had to decide whether the suspect in the accompanying scenario was guilty based on the evidence. They wrote a CER and then talked through argumentation as a class. The endangered species activity is presented in this article.