The answer is a portable body warmer, “hokkairo” in Japanese.

　

”Kairo” as the first portable body warmer was invented when there were Samurais in Japan about 400 years ago.

At that time, they put ashes into a sealed metallic case and burn them. And in 1978, a modern disposable body warmer was invented.

This size is about flat of our hand and we can carry on it in our pocket.

There are also smaller ones, as small as a calling card, and a large ones, as big as a postcard.

We often carry it to warm ourselves when we go out in cool season.

How to use it is as follows: we just shake it and it gets rather warm in an instant.

This disposable body warmer is a bag of pouched bonded material.

And it holds active carbon, iron powder and so on inside.

They combine with oxygen and become warm.

In general, it keeps for about 12 hours to 14 hours. Some has a sticky material pasted on one side and you can paste it directly on to your skin and it keeps warm much longer.

But they have dangerous aspect. If we keep putting it directly to our skin at a same place for a long time, it might cause burn.