  Neutrino physics is used as an illustrative example for an elementary
introduction to the computational method of Feynman loop-diagrams and decay
rates. If the neutrinos are as massive as recently reported by the
Super-Kamiokande results, the heaviest neutrino $\nu_H$ would not be stable.
Although chargeless, it could decay -- by quantum loop effect -- into a lighter
neutrino $ \nu_L$ by emitting a photon: $\nu_H \to \nu_L + \gamma $. If
kinematically possible, the $\nu_H \to \nu_L$ + e$^+$ + e$^-$ mode could occur
at the tree diagram level and furthermore get enhanced, at one-loop radiative
corrections, by a large logarithm of the electron mass acting as an infrared
cutoff.
