The hot water bag is commonly used by menstruating women. The design is inspired by an angry pufferfish, which is likened to a grumpy woman during menstruation. When hot water is poured into the bag, its volume, the length of its stinger and its expression change to indicate that the angry puffer fish becomes stable under male care. This change is meant to remind men to pay more attention to women's emotional well-being during their periods.