In classical physics, there are three states in which matter occurs - solid, liquid, and gas. In a solid, the molecules are closely bound to one another by molecular forces. A solid holds its shape and the volume of a solid is fixed by the shape of the solid. In a liquid the molecular forces are weaker than in a solid, and it will take the shape of its container while its volume remains fixed. In a gas the molecular forces are very weak. A gas fills its container, taking both the shape and the volume of the container. Plasma, the collection of charged gaseous particles, is sometimes called the fourth state of matter.