Brake fluids for normal operation are glycol-based fluids that transfer pressure in the vehicle's hydraulic braking system. When the brake pedal is depressed, the fluid transfers the force to the brakes at the wheels and deceleration occurs. The most commonly used types are those labelled DOT 3, DOT 4 and DOT 5.1. The main characteristic of these fluids is their so-called hygroscopicity, which means that they bind moisture in the air. Over time, the proportion of water in the liquid increases, which lowers its boiling point. Intense braking generates heat that can bring a liquid with a low boiling point to a boil. The resulting gas bubbles are compressible and the braking system then loses efficiency. Individual specifications differ precisely in the amount of dry and wet boiling points, with a higher number generally indicating a higher boiling point. For this reason, brake fluid must be changed regularly, usually at two-year intervals.