Gases and plasmas undergo changes in volume most easily. In contrast, liquids and solids have fixed volumes, although liquids do not have fixed shapes. Gases and plasmas expand or contract to fill their containers.
Liquids and solids have molecules that are tightly bonded together, limiting movement. Molecules can slide around each other in liquids, but they are unable to move freely. This results in liquids being able to flow, although they maintain fixed volume. Molecules are locked together in solids, preventing any changes in either volume or shape. Molecules in liquids and gases are separated, and they are able to move freely, allowing those forms of matter to change their volumes very easily.