A heating device has a receiving space for holding water and a flexible wall. An inorganic phase change material for at least temporarily controlling a temperature of the water is disposed in the receiving space. The phase change material has a property of absorbing energy during an endothermic phase transition when warmed by the water and releasing energy in the form of heat during an exothermic phase transition. The phase change material may partially or completely undergo the endothermic phase transition when hot water is poured into the receiving space. The phase change material is disposed in a housing formed as a flexible film composed of a polymer material. An actuating means for triggering the exothermic phase transition is in fluid communication with the phase change material.