Glasses are solid materials without crystalline molecular order. Unlike “traditional” solids, the atoms and molecules in these materials are not packed in a regular grid-like pattern, but are arranged in a more “random”, disordered fashion. Almost all materials can be prepared in this form by cooling their melts at a fast enough rate. For metal alloys, this requires cooling rates in excess of a million degrees per second. For polymers, however, the required cooling rates are much slower, and many polymers can be obtained in glassy form by simply letting their melts cool in air. The same applies to common window glass, which is composed of silica with certain impurities.
            Glasses are different from melts in that they behave like solids when observed over short times (seconds or minutes). At longer time scales (days, years), they show more and more liquid-like behavior. Upon cooling a melt, the transition to glassy behavior does not occur gradually, but quite sharply at a certain temperature, known as the glass transition temperature. Many aspects of glasses and the glass transition are still poorly understood, and represent an active field of current research.