Three components must be present in order for a fire to occur: oxygen (or an oxidizing agent), heat and fuel. This is often represented by the "fire triangle" or "combustion triangle." The fire tetrahedron represents the addition of a component, the chemical chain reaction, to the three already present in the fire triangle. After the fire ignites, an exothermic chain reaction keeps the fire going until something is introduced into the fire that blocks it. Water is one of the most common choices of fire suppressants. But in some cases, water will not suppress the fire, and may even make it worse. Flame retardant chemicals were developed to block the other elements that were part of the "fire triangle." Flame retardant chemicals may block the oxygen (foams), hydrated flame retardant chemicals release water (aluminum trihydrate and magnesium hydroxide) and halogenated flame retardant chemicals (bromine and chlorine) remove free radicals and create a barrier of inert gas that suppresses the chemical reaction creating the fire.