The heating of the boiler, "A", leads to a gradual increase of the pressure due to both the expansion of the enclosed air and the raised vapor pressure of the increasingly heated water. When pressure becomes high enough to force the water up the funnel through the coffee grinds, coffee will begin to pour into the upper chamber (C). Boiling the water is not necessary - or even desirable - to produce sufficient pressure to brew and extraction temperature is in general not greater than that of other brewing methods.