A hot water bag is used to apply heat to parts of the body because water has a high specific heat capacity (\(\rm 4200 Jkg^{-1}K^{-1} \)) , meaning it can store and retain heat for a long time.

 When hot water is put into the bag, it slowly releases heat, which helps to soothe sore muscles, reduce stiffness, and improve blood circulation in the area. This gradual release of warmth provides consistent, gentle heat, making it effective for pain relief and relaxation.