The inner side of these lunch boxes is made of food-grade stainless steel material. The steel helps in retaining the heat of the food. It also retains the taste and nutritive value of the contents for a longer duration. It is durable and food-safe.These lunch boxes have a spill-proof design which brings down the chances of spillage or leakage of the oil or gravies. The lids have a corner that juts out of the frame to allow easy opening of the containers.