An "amorphous solid" is a solid in which there is no long-range order of the positions of the atoms. (Solids in which there is long-range atomic order are called crystalline solids or morphous). Most classes of solid materials can be found or prepared in an amorphous form. For instance, common window glass is an amorphous solid, many polymers (such as polystyrene) are amorphous, and even foods such as cotton candy are amorphous solids. In principle, given a sufficiently high cooling rate, any liquid can be made into an amorphous solid. Cooling reduces molecular mobility. If the cooling rate is faster than the rate at which molecules can organize into a more thermodynamically favorable crystalline state, then an amorphous solid will be formed. Because of entropy considerations, many polymers can be made amorphous solids by cooling even at slow rates. In contrast, if molecules have sufficient time to organize into a structure with two- or three-dimensional order, then a crystalline (or semi-crystalline) solid will be formed. Water is one example. Because of its small molecular size and ability to quickly rearrange, it cannot be made amorphous without resorting to specialized hyperquenching techniques. Amorphous materials can also be produced by additives which interfere with the ability of the primary constituent to crystallize. For example, addition of soda to silicon dioxide results in window glass, and the addition of glycols to water results in a vitrified solid. Some materials, such as metals, are difficult to prepare in an amorphous state. Unless a material has a high melting temperature (as ceramics do) or a low crystallization energy (as polymers tend to), cooling must be done extremely rapidly. As the cooling is performed, the material changes from a supercooled liquid, with properties one would expect from a liquid state material, to a solid. The temperature at which this transition occurs is called the glass transition temperature or Tg.