It is a hot-water bottle or bag. We put it inside futons to make the legs or bodies warm. The original model of the modern-day hot-water bottle is the one came from China in the Muromachi era and made of clay or metals. These days, the commonly-used ones are made from plastic and there are also several types: a rechargeable one and the reusable one with a microwave. The reason for its popularity is that it can retain natural heat without electricity.