You can cover it or leave it uncovered. The purpose of covering it is to prevent water loss and oil from being measured outside. If it is a normal meal, it can be heated directly without cover. Some foods that are easy to fry and jump out can be covered with a lid, but the lid must have air holes and cannot be completely sealed, otherwise the bowl will easily explode. If the cover with buckle can be hung on the bowl without pressing, the edge of the bowl can breathe or the air hole on the cover can be opened to let the air breathe.