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In this activity, student teams will create a controlled experiment by building ecosystems in two 2-liter bottles. Teams determine the control conditions of both bottles, identify a test variable, and run the experiment for several weeks to determine the role of the test variable in the system. The experiment uses living organisms such as aquatic plants or pond water microorganisms, a pH and ammonia test kit, light source and thermometer. Questions assessing understanding are provided. This resource is supported by Chapter 1, "What is Global System Science?" part of Global Systems Science (GSS), an interdisciplinary course for high school students that emphasizes how scientists from a wide variety of fields work together to understand significant problems of global impact.