A simple rotating system with no external forces acting on it carries a fixed angular momentum and an associated rotational kinetic energy. If the system changes its internal configuration, such as a spinning skater retracting or extending his/her arms, the angular momentum remains constant since no external torque is applied on the system. However, the rotational kinetic energy changes with the configuration. How is this possible when there is no external work done involved? Where does the change of the rotational energy go or come from?