Relationship between boiling point and air pressure of liquid. The boiling point of liquid decreases when the air pressure decreases and increases when the air pressure increases. This law is applicable to all liquids. At an altitude of 3Km, the boiling point of water is 91℃; In a mine 600m underground, the boiling point of water is 102℃.

Working principle of pressure cooker. The pressure cooker seals the water quite tightly, and the steam generated by water evaporation cannot diffuse into the air, but only remains in the pressure cooker, so that the pressure inside the pressure cooker is higher than the external atmospheric pressure.

Heat conduction. A process in which the particles of a large number of substances collide with each other in thermal motion, so that energy is transferred from the high-temperature part of the object to the low-temperature part or from the high-temperature object to the low-temperature object. The greater the temperature difference, the faster the heat conduction speed.

On the whole, because the pressure cooker can produce pressure higher than the atmospheric pressure of life, the liquid can produce a temperature higher than that which can not be achieved normally, and the temperature of the liquid (soup or water) increases, and the speed of heat transfer to the food will be accelerated, so the food will be cooked more easily.