Specifically, once the water in lower chamber is hot enough that its vapor pressure (the pressure exerted by the vapour component of a liquid) exceeds the pressure of a standard atmosphere, some of it begins to boil, turning into water vapor. Since the density of water vapor is about 2000 times smaller than that of liquid water, the mixture of the air and water vapor in the lower chamber quickly expands, and, when the new pressure exceeds atmospheric pressure, pushes the remaining water up the siphon tube into the upper chamber, where it remains so long as the pressure difference between the upper and lower chambers is sufficient to support it (about 1.5 kPa or 0.015atm). This pressure difference is maintained during brewing through the continually heating of the lower chamber. Coffee grinds are added to the water in the upper chamber.