The atoms and molecules of most substances in their solid state arrange themselves in a certain structure from the intrinsic nature of the forces acting against each other: The structure in which the total of the energies of all bonds is minimised. This structure is referred to as the crystal structure of the substance. Most solid substances are therefore present in solid form as crystals. Atoms or molecules can only be released from this crystal structure by supplying considerable amounts of energy, usually the energy required to melt the substance. In most cases, however, a substance is not present as a uniform crystal. If, as usual, solidification starts independently in many places in a substance, a large number of crystal nuclei form which prevent the formation of a single crystal ("mono-crystal"). This is why most substances are polycrystalline, so that a crystal structure is not recognizable on a macroscopic scale. To generate a mono-crystal, very particular growth conditions must be provided which prevent the formation of more than one crystal nucleus. Solid substances in which atoms and molecules are present not in a crystalline arrangement but are randomly distributed are referred to as amorphous. In many cases, in particular with plastics , slow cooling can be used to achieve an almost purely crystalline structure, while abrupt cooling, leaving the molecules no time to arrange themselves in a specific order, can be used to achieve an amorphous structure.