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. A collision between two molecules is similar to a collision between two balls. The molecules exchange momentum, but the total momentum of the two molecules is conserved .