  Einstein's relativity theory appears to be very accurate, but at times
equally puzzling. On the one hand, electromagnetic radiation must have zero
rest mass in order to propagate at the speed of light, but on the other hand,
since it definitely carries momentum and energy, it has non-zero inertial mass.
Hence, by the principle of equivalence, it must have non-zero gravitational
mass, and so, light must be heavy. In this paper, no new results will be
derived, but a possibly surprising perspective on the above paradox is given.
