The strength of the solid depends on the strength of the bonds that hold the molecules together. Liquids, obviously, do not retain a fixed shape, but they do have a definite volume. Gases, on the other hand, have no fixed shape or volume. Rocks do so when superheated in the center of the Earth. A fourth state of matter, called plasma, exists when a gas becomes ionized.
Plasma exists inside stars and in interstellar gases.
An recently scientists have theorized about a fifth state of matter, called a Bose-Einstein condensate. Scientists describe it as the opposite of a plasma. This fifth state occurs at super low temperatures, near where atoms are not moving at all. The atoms settle into their lowest possible energy state and begin to share some of their physical characteristics with those of surrounding atoms. The Exploratorium has a cool experiment on this topic.