Without going too much into the science, water is mixed with �an antifreeze� � which means a chemical agent that increases the boiling point of the water. Increasing the water�s boiling point also decreases the point at which it freezes because it develops a greater capacity for taking on and transferring heat. The idea here is to make sure that the coolant stays in its liquid form � because if it freezes it�ll expand (breaking any rigid mechanics in the pump). There are many variations of additives, but the most common ones are ethylene glycol and propylene glycol.