Habit problem. After World War II, Japan experienced a period of rapid revival. At that time, people were overworked and it was easy to save a lot of time for eating. They cooked lunch boxes, such as octopus ham, rice, dried plums, egg rolls, seaweed leaves and so on. Just for cold food. There are many cold meals in Japanese food. Like sushi, I haven't eaten anything too hot! Generally speaking, most of their lunches will be sushi. And some raw food, of course, cold food. If you heat them, they won't taste like that! Try it, it's delicious!