Solid, liquid, and gas are the three familiar phases in which matter is normally found on Earth. Solids are rigid and maintain their shapes and volumes, liquids have fixed volumes but change to fit the shape of their containers, and gases expand to fill the full volume of their containers.
The phase of an element or chemical depends on its temperature and the pressure it is subjected to. At cold enough temperatures and atmospheric pressure, matter usually is in the solid phase. If it is heated sufficiently, the solid melts into a liquid, and further heating leads to evaporation into a gas. On Earth, water comes naturally in all three phases as ice, liquid water and atmospheric water vapor.