  We ponder about neutrino oscillations; a particle without a clear identity, a
neutrino of a given flavor in fact does not satisfy the Dirac equation (which
is used to define the mass eigen-states and mass eigen-values). This alters the
basic treatment of neutrino oscillations, in that the Dirac spinors are defined
as the mass eigen-states while the flavor states can only be given as linear
combinations of the mass eigen-states (of Dirac equations). Even though
neutrino masses are tiny, the impacts of several neutrino masses in a single
reaction or a single decay, such as possible violation of the energy-momentum
conservation, should not be overlooked.
  Among those sources of oscillating neutrinos, we point out that the ultra
high energy cosmic rays (UHECR's) such as the proton of energy $10^{18}\, eV$
or higher, used to think of being rather stable, can capture, in the matter
media, an electron to convert into an electron-like neutrino and a spectator
neutron. This would be the most important neutrino source of the UHECR origin.
