This lesson, provided by Science NetLinks, is the fourth of a sequence of five lessons, teaching students that most of the materials and energy used by a city come from outside the city boundaries. Students will need to have at least a general working understanding of the concepts of flow (as in energy flow) and cycles (as in nutrient cycles) in order to get the most out of this lesson. The class will use various websites to investigate these processes and then create dramatic or musical skits demonstrating their understanding of the “”actors”” in the urban metabolism.