  Since the appearance of Einstein's paper {\em"On the Electrodynamics of
Moving Bodies"} and the birth of special relativity, it is understood that the
theory was basically coded within Maxwell's equations. The celebrated
mass-energy equivalence relation, $E=mc^2$, is derived by Einstein using
thought experiments involving the kinematics of the emission of light
(electromagnetic energy) and the relativity principle. Text book derivations
often follow paths similar to Einstein's, or the analysis of the kinematics of
particle collisions interpreted from the perspective of different inertial
frames. All the same, in such derivations the direct dynamical link with
hypothetical fundamental fields describing matter (e.g. Maxwell theory or
other) is overshadowed by the use of powerful symmetry arguments, kinematics,
and the relativity principle.
  Here we show that the formula can be derived directly form the dynamical
equations of a massless matter model confined in a box (which can be thought of
as a toy model of a composite particle). The only assumptions in the derivation
are that the field equations hold and the energy-momentum tensor admits a
universal interpretation in arbitrary coordinate systems. The mass-energy
equivalence relation follows from the inertia or (taking the equivalence
principle for granted) weight of confined field radiation. The present
derivation offers an interesting pedagogical perspective on the formula
providing a simple toy model on the origin of mass and a natural bridge to the
foundations of general relativity.
