A crystal is a substance in which a substance's particles (molecules, atoms, and ions) are regularly and repeatedly arranged in a three-dimensional space. From a macroscopic point of view, crystals have their own unique, symmetrical shapes, such as cubes of salt; hexagonal prisms of ice; and octahedrons of alum. Crystals have different physical properties in different directions, such as mechanical strength, thermal conductivity, thermal expansion, electrical conductivity, etc., and are called anisotropic. Crystals have a fixed melting temperature—melting point (or freezing point). Crystals are widely distributed, and most of the solids in nature are crystals. Gas, liquid, and amorphous materials can also be converted to crystals under certain suitable conditions.