  This paper describes a particularly transparent derivation of the Hawking
effect for massive particles in black holes. The calculations are performed
with the help of Painleve-Gullstrand's coordinates which are associated with a
radially free-falling observer that starts at rest from infinity. It is shown
that if the energy per unit rest mass, e, is assumed to be related to the the
Killing constant, k, by k = sqrt(2e -1) then e, must be greater than 1/2. For
particles that are confined below the event horizon (EH), k is negative. In the
quantum creation of particle pairs at the EH with k = 1, the time component of
the particle's four velocity that lies below the EH is compatible only with the
time component of an outgoing particle above the EH, i.e, the outside particle
cannot fall back on the black hole. Energy conservation requires that the
particles inside, and outside the EH have the same value of e, and be created
at equal distances from the EH, (1 - rin = rout - 1). Global energy
conservation forces then the mass of the particle below the EH to be negative,
and equal to minus the mass the particle above the EH, i.e, the black hole
looses energy as a consequence of pair production.
