Cities are like giant rock formations made by humans, using materials like steel, glass, and concrete. These materials are brought in and out of cities every day, creating a kind of urban metabolism. Instead of natural forces like climate shaping these rock formations, humans control the flow of materials using fossil fuels. These man-made rocks are evolving quickly, especially in terms of waste. This new way of looking at cities can help us understand how they will need to change in the future.