  It has been firmly established, that neutrinos change their flavour during
propagation. This feature is attributed to the fact, that each flavour
eigenstate is a superposition of three mass eigenstates, which propagate with
different frequencies. This picture, although widely accepted, is wrong in the
simplest approach and requires quite sophisticated treatment based on the
wave-packet description within quantum field theory. In this communication we
present a novel, much simpler explanation and show, that oscillations among
massive particles can be obtained in a natural way. We use the framework of
quantum mechanics with time being a physical observable, not just a parameter.
