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 Painlevé-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 Killing constant, k, by k2 = 2e – 1 then e, must be greater than ?. 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 has the same value of e, and is created at equal distances from the EH, (1 – rin = rout – 1). Global energy conservations force 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.