The role of water in extinguishing a fire can be summarized as follows: The main effect is cooling down the fire by absorption of heat energy either at the fuel surface or in the gas phase. A contributing effect is diluting the atmosphere by adding vapor and thereby removing oxygen from the fire The main limits to the use of water are directly linked to the physical-chemical characteristics of water itself: Water can't be used on certain type of fires such as (1) Fires where live electricity is present- as water conducts electricity it presents an electrocution hazard; (2) Hydrocarbon fires - as it will only spread the fire because of the difference in density; (3) Metal fires - as these fires produce huge amounts of energy (up to 7. 550 calories/kg for Aluminum) and water can also create violent chemical reaction with burning metal (by oxidization); (4) Fat fires - as vapor will carry and spread burning oil everywhere. Since these reactions are well-understood, it has been possible to create specific water-additives which will allow: (1) A better heat absorption with a higher density than water; (2) Carrying free radical catchers on the fire; (3) Carrying foaming agents to enable water to stay on the surface of a liquid fire and prevent gas release and (4) Carrying specific reactives which will react and change the nature of the burning material. Water-additives are generally designed to be effective on several categories of fires (class A + class B or even class A + class B + class F), meaning a better global performance and usability of a single extinguisher on many different types of fires (or fires that involve several different classes of materials)