Most of fire is burning gas vapors. The vapors are released from the fuel in proportion to the vapor pressure of the fuel, which is dependent on the material and heat. If heat is removed, less vapors are emitted, and the fire is reduced. So how is heat removed? Water has a tremendous heat absorption capability. As water absorbs heat, temperature then goes down, and less vapor is released from the fuel. The ideal gas law, PV=nRT shows that pressure and volume of the gas is related to temperature, so by reducing temperature, one can reduce the energy in the pressure and amount of fuel vapor (volume), and the fire can be contained.
