Stainless steel lunch boxes should not be heated in the microwave oven. Stainless steel belongs to metal. When the metal utensils put into the microwave oven are heated, they will generate electric sparks and reflect microwaves, which will not only damage the furnace body but also heat food. If a metal object is heated in a microwave oven for a long time, it is likely to spark. Sparks appear frequently, which may ablate the metal wall, and microwaves cannot be generated normally. It may also cause the microwave oven to malfunction and the service life of the microwave oven to be shortened. Metal itself is not the cause of sparks, on the contrary, the shape of metal is the culprit. Metals with sharper edges are more likely to cause sparks, while objects with circular edges are relatively safe.