Now to the details. Boiling is the spontaneous change of water from the liquid phase to the gas phase. However, as is true for all materials, temperature is not the only parameter that determines the phase; there is also pressure that affects its phase. Thus, every material has a "phase diagram", with temperature and pressure as its coordinates.

Although water has some strange behaviour at the liquid-solid boundary, the liquid-gas boundary is quite standard. The boiling point drops with decreasing pressure. The familiar 100°C (212°F) is the boiling point of water at 1 atmospheric pressure (i.e., the mean pressure at sea level). When the pressure drops, the boiling point drops as well.

So, if you want to boil water, you have two options:

1. Add heat until the temperature reaches the boiling point at the fixed pressure.

2. Reduce pressure (e.g. by vacuuming the container water is in) until the boiling point at that pressure drops below the fixed temperature.