DOT-5.1 is one of several designations of automotive brake fluids, denoting a particular mixture of chemicals imparting specified ranges of boiling point. A brake fluid's dry boiling point is the temperature at which the pure fluid will boil. Wet boiling point concerns the boiling temperature when the fluid contains water. DOT-5.1, like DOT-3 and DOT-4, is a polyethylene glycol-based fluid (contrasted with DOT-5 which is silicone-based). Polyethylene glycol fluids are hygroscopic and will absorb water from the atmosphere, which is necessary to prevent sheer and undiluted water in the braking system, which is very corrosive. Also water droplets can freeze in the pipes, thus blocking the system.