Three phases of matter

In classical Physics there are three phases of matter, the solid phase, the liquid phase and the gas phase. The atoms in a solid sample are in a fixed position to one another. Solids are not compressible. The atoms in a liquid sample are not in a fixed position to one another, but they are at a density from one another. Like solids, liquids are not compressible, but liquids do not have a fixed shape. They can flow, and they will fill the bottom of any container they are put in. A sample of gas is compressible, does not have a fixed density, and will expand to completely fill any container it is put in.

Modern Physics recognizes other exotic states of matter, including plasma, superfluids, and Bose-Einstein condensate.