Solids usually consist of a simple geometric pattern of atoms or molecules which is repeated over and over again in all directions, which is known as a crystal . Metals therefore conduct electricity very easily. Liquids also contain closely packed molecules, but they do not have a rigid orientation. They still feel an electrostatic attraction, and they regularly bump into each other, but they can move around with relative ease. A gas whose atoms have been ionized into a mixture of positive ions and electrons is called a plasma .
Stars consist of plasmas.
Liquids, gases, and plasmas are all referred to as fluids , because they can flow and take the shape of whatever container they are placed in. Generally, a solid is denser than a liquid, because it packs its molecules into a smaller volume. Similarly, liquids are denser than gases. The molecules which make up solids, liquids, and gases all move around to a lesser or greater extent. In rigid solids, the molecules can vibrate back and forth, and also rotate slightly.