Recent studies of American science education have highlighted the need for more inquiry-based lessons. For example, when the National Research Counsel evaluated the Advanced Placement (AP) Biology program, it pointed out, "AP laboratory exercises tend to be "cookbook" rather than inquiry based. This criticism is particularly apt for the lab exercises on photosynthesis and cellular respiration recommended by the College Board and others. In this article, the author describes an alternative exercise that simultaneously addresses photosynthesis and cellular respiration using an inquiry-based approach free of technical complexity or costly instrumentation. The author and his class began with a mystery. (Contains 3 figures and 8 footnotes.)