A fire retardant is a substance that reduces flammability of fuels or delays their combustion. This includes chemical agents, but may also include substances that work by physical action, such as cooling the fuels, such as fire-fighting foams and fire-retardant gels. Fire retardants may also be coatings applied to an object, such as a spray retardant to prevent Christmas trees from burning. Fire retardants are commonly used in fire fighting.