Phase change materials (PCMs) are substances that absorb or release heat when they change from one state to another, such as from solid to liquid or vice versa. This property allows them to store thermal energy, making them useful in various applications, including building temperature regulation and thermal energy storage systems. When PCMs melt, they absorb heat, which helps to cool the surrounding environment. Conversely, when they solidify, they release heat, providing warmth. Common examples of PCMs include paraffin wax and salt hydrates, which are often used in energy-efficient buildings and temperature-controlled packaging.