A solid can be defined as a substance which exists in the solid-state, which is one of the four fundamental states of matter. Solids feature closely packed atoms whose kinetic energies are much lower than those of liquids and gases. 

Liquids have the following characteristics: No definite shape (takes the shape of its container). Has definite volume. Particles are free to move over each other, but are still attracted to each other. 

Gas is one of four natural states of matter, along with liquid, solid and plasma. A gas has no fixed shape or volume. The atoms or molecules that make up the gas fill the container that holds them.