AAAS: THE PHYSICAL SETTING: ENERGY TRANSFORMATIONS - Heat energy in a material consists of the disordered motions of its atoms or molecules
Heat energy in a material consists of the disordered motions of its atoms or molecules. In any interactions of atoms or molecules, the statistical odds are that they will end up with less order than they began-that is, with the heat energy spread out more evenly. With huge numbers of atoms and molecules, the greater disorder is almost certain.