Melting is the physical process where a substance goes from a solid state to a liquid state. Scientists refer to this as a "phase change." Normally, this phase change occurs at a particular temperature and pressure.
Matter on Earth exists in one of three common phases: solid, liquid or gas. Water, for example, can exist as ice, liquid water or vapor. If a solid, even a dense solid such as iron, is heated to a high enough temperature, it will melt into a liquid. If head continues to be added, the liquid will eventually vaporize into a gas. The processes work in reverse as well. Cooling a gas enough condenses it into a liquid. Cooling it even further solidifies the iron. In all cases the addition of heat (energy) increases the mobility of the molecules. The more they move, the less they stick together. Solid form is the lowest energy arrangement that the molecules can have. In a solid, the molecules tend to exist in a stable crystal lattice. In a gaseous form, the molecules are so energized that they cannot stay together and tend to fly off in random directions. A gaseous state is the least organized phase. The liquid state is the natural state of water on Earth between the temperatures minus 32 F — its freezing point — and 100 F — its boiling point.