Boila

Meet Boila, a pot of water that bubbles sooner when the air presses less.

Water molecules are always escaping from a liquid's surface, creating vapour pressure, which rises steeply with temperature. Boiling starts when bubbles of vapour can form inside the liquid and survive, which happens once the vapour pressure equals the pressure pushing down from outside. At sea level, about 101 kPa, that happens at 100 °C. Lower the outside pressure and the water reaches it at a lower temperature.

High in the mountains the air pressure is much lower. Near the top of Mount Everest it is roughly a third of sea-level pressure, and water boils at around 70 °C, so food cooks slowly even though the pot is bubbling hard.

Pressure cookers use the same rule in reverse: they trap steam to raise the pressure to about twice atmospheric, so the water stays liquid until about 120 °C and food cooks faster.

A common misconception is that boiling water is always at 100 °C. It is only at 100 °C at standard atmospheric pressure; dissolved salt also raises the boiling point slightly.