This simple activity will help students to understand heat transfer. Heat energy is a form of energy that is present in the molecules and atoms of an object. When an object is hot, its molecules have a lot of energy and therefore move fast. When an object is cold, the molecules begin to slow down thus decreasing the overall amount of energy. The faster the molecules are moving, the more space they take up. Heat is the transfer of kinetic energy from one medium or object to another or from an energy source to a medium or object. The standard unit of heat in the International System of Units (SI) is the calorie (cal), which is the amount of energy transfer required to raise the temperature of one gram of pure liquid water by one degree Celsius, provided the water temperature is higher than the freezing point and lower than the boiling point.