||In prior grades students learned to think systematically about how the parts of objects, plants, and animals are connected and work together. In grades 4-5 students learn that systems contain smaller (sub-) systems, and that systems are also parts of larger systems. The same ideas about systems and their parts learned in earlier grades apply to systems and subsystems. In addition, students learn about inputs and outputs and how to predict what may happen to a system if the system's inputs are changed. The concept of a hierarchy of systems provides a conceptual bridge for students to see the connections between mechanical systems (e.g., cities) and natural systems (e.g., ecosystems).