You can't. Metals can only reflect microwaves, but cannot penetrate and absorb them. If you put the stainless steel lunch box in the microwave oven, the food inside can't be heated. Metal utensils are not suitable for microwave heating, because the utensils such as iron, aluminum, stainless steel and enamel put into the furnace will generate electric sparks when heated, which will not only damage the furnace body, but also heat immature food.

Don't use metal utensils for microwave heating: Because iron, aluminum, stainless steel, enamel and other utensils put into the furnace will generate electric sparks when heated in the microwave oven, which will not only damage the furnace body, but also heat immature food. Avoid using closed containers: when heating liquid, use wide-mouth containers, because the heat generated by food heating in closed containers is not easy to dissipate, which makes the pressure in the containers too high and easily leads to blasting accidents. When cooking food with a shell, you should also pierce the shell with a needle or chopsticks in advance to avoid bursting after heating, splashing out to pollute the furnace wall or hurting people.