“Everything in Earth's system can be placed into one of four major subsystems: land, water, living things, or air. These four subsystems are called "spheres." Specifically, they are the "lithosphere" (land), "hydrosphere" (water), "biosphere" (living things), and "atmosphere" (air).”
From:
“Earth’s Sphere.” Classroom of the Future. November 10, 2004. www.cotf.edu/ete/ess/essspheres.html.
