lithosphere: You may not have heard this word before, but it refers to something that you are very familiar with: it is the solid crust of the planet. The lithosphere is where we live, the ground that we walk on; it is the soil and rocks, the mountains and valleys. It forms the continents and islands and even the ocean floors.

hydrosphere: You can probably guess what this term means, since you may know that hydro means water. The hydrosphere is all of the earth's water, including the oceans, seas, lakes, rivers, streams, ice, even the clouds and fog and rain.

biosphere: This sphere is different from the other three spheres and it is also a part of all the other spheres. The biosphere is the sphere that includes all life forms. All living things, including organisms in the far reaches of the atmosphere and in the deepest part of the ocean, are a part of the biosphere. You are a part of the biosphere.

atmosphere: This is probably the term you are most familiar with: it is the air that you breathe. The atmosphere is made of all of the gases that surround the earth. It starts on the earth's surface and extends into space. As you might guess, it is the least dense of all the four spheres.