5 mm or more. The process of steaming steamed bread produces a lot of steam, and the lid is too tightly sealed, which leads to the backflow of distilled water and scalding the steamed bread to death. It is necessary to make an air outlet above 5 mm. The lid is generally round with a knob in the middle. In ancient times, the texture of the pot cover was mostly wood, generally metal, and aluminum was common. In Yunnan, there is also a straw pot cover similar to a straw hat. The main function of the pot cover is to keep the temperature of the substance in the pot and prevent the liquid splashing and steam escaping caused by heating and other factors.