Distilled water is water that has been boiled into vapor and condensed back into liquid to remove all impurities, minerals, and bacteria. In automotive applications, it is the only type of water that should be mixed with antifreeze concentrate for the cooling system. Using tap water introduces minerals that cause scale buildup (limescale) inside the radiator and engine block, leading to poor heat transfer and overheating. While similar to battery water, it is also safe for use in cleaning sensitive electronics.