Einstein's mass–energy equivalence principle, E = mc², where E is energy, m is mass, and c is the speed of light, is probably the best-known formula in science. Despite its algebraic simplicity, however, it demonstrates an inherent fundamental difficulty for direct experimental verification: The comparison between energy and mass is realized by a factor of c², which magnifies any mass variation by 17 orders of magnitude. Accordingly, the energy equivalent of 1 kg (2.2 lb) of mass is comparable to the energy released by the largest nuclear explosion achieved so far. Therefore, the choice of physical system for experimental verification of Einstein's formula is limited to microscopic masses. Here the thermal neutron capture reaction appears to be the best candidate: A thermal neutron with a kinetic energy of a few millielectronvolts (meV) induces a nuclear reaction in which a new isotope with excitation energy of the order of 10 MeV is formed. The relative energy uncertainty of this reaction is therefore of the order of 10⁻⁹, which would allow carrying out very precise experiments.