Tempered glass is glass that is heat-treated. Glass is first heated and then the surfaces are cooled rapidly. The edges cool first, leaving the center relatively hot compared to the surfaces. As the center cools, it forces the surfaces and edges into compression. Tempered glass is approximately four times as strong as annealed glass. It is able to resist temperature differences of 200°F–300°F (90°C–150°C). Since it resists breakage it is favored in many applications for its safety characteristics such as automobiles, doors, tub and shower enclosures, and skylights.