  One of the most widespread interpretations of the mass-energy equivalence
establishes that not only can mass be transformed into energy (e.g., through
nuclear fission, fusion, or annihilation) but that every type of energy also
has mass (via the mass-energy equivalence formula). Here, we show that this is
not always the case. With the help a few thought experiments, we show that, for
instance, the electric potential energy of a charged capacitor should not
contribute to the capacitor's gravitational rest mass (while still contributing
to its linear momentum). That result is in agreement with the fact that light
(ultimately, an electromagnetic phenomenon) has momentum but not rest mass.
