 Using a common heuristic in relativistic physics, one considers the physical interaction first in the inertial reference frame in which \(B\) is at rest, which is sometimes called the “rest frame.” One then compares the mathematical description of this interaction to the corresponding description from the perspective of a different inertial reference frame that moves with a constant velocity relative to the rest frame. Finally, using dynamical principles, such as the conservation of energy or conservation of momentum, and the two principles at the core of special relativity (i.e., the relativity principle and the light principle), one shows that the dynamical principles require that after \(B\) suffers the interaction in which it absorbs or emits energy, its mass (i.e., rest-mass) must change by an amount given by Einstein’s equation. 