The relationship between temperature and volume, at a constant number of moles and pressure, is called Charles and Gay-Lussac's Law in honor of the two French scientists who first investigated this relationship. Charles did the original work, which was verified by Gay-Lussac. They observed that if the pressure is held constant, the volume V is equal to a constant times the temperature T
When you are heating food in a closed container, the container will open in order to release the pressure inside it. If it doesn't, the container will quickly release all the pressure, which can be dangerous because the gases inside the container may be heated.