In a solid , the atoms and molecules are densely packed and held in place by intermolecular forces. The atoms in a solid can be modeled as being held together by tiny springs that permit them to vibrate back and forth about their equilibrium position, but not to exchange positions with other atoms. Solids are nearly incompressible . In a liquid the atoms and molecules are also densely packed. They cannot easily escape from one another, but they are free to move with respect to each other.
Liquids are nearly incompressible.
In a gas , intermolecular forces are weak and short ranged, and the atoms and molecules can move about nearly independently. Gases are compressible . Gases and liquids are fluids , i.e. collections of atoms or molecules that are free to move with respect to each other. Assume we have a collection of gas molecules in a container in gravity-free space. Then each molecule moves along with constant velocity in a straight line, until it hits another molecule, or a container wall.