While conventional cooking is limited to an effective maximum temperature of the boiling point of water, the pressure cooker allows us to raise that maximum from 212 to 250°F/100 to 120°C. It does so by tightly sealing the meat and cooking liquid in the pan and allowing the vaporizing water to build up the pressure to about double the normal air pressure at sea level. This increased pressure increases the boiling point, and high pressure and temperature put together produce an overall doubling or tripling of the heat transfer rate into the meat, as well as an extremely efficient conversion of collagen into gelatin. Pot roasts cook in less than an hour instead of two or three. Of course the proteins get very hot and therefore squeeze out much of their moisture; meat must be well endowed with fat and collagen to end up anything but dry.