Matter exists in three primary physical states: solid, liquid, and gas. In a solid, particles are closely packed together, giving it a definite shape and volume. Solids do not flow and maintain their form unless a force is applied. In a liquid, particles are still close but can move past one another, allowing liquids to take the shape of their container while maintaining a constant volume. In a gas, particles are far apart and move freely, filling the entire space available. Gases have neither a definite shape nor a fixed volume.