Due to a concept called specific heat.Specific heat of a substance is the amount of heat energy absorbed or lost by that substance to change its temperature.Now, the hot water bag consists of hot water.As compared to other liquid like milk and acetone, the specific heat of water is much higher. This means that as compared to others.Water needs to absorb a large amount of heat to become hotter.Now, did you know that water will take a long time to cool down as comapred to milk and acetone?Thus, water will take a longer time to become cool.

Hence, it proves to be very useful in hot water bags because the warmth can be utilized for a longer time.