  We consider the decay of a particle with some energy $E_{0}>0$ inside the
ergosphere of a black hole. After the first decay one of particles with the
energy $E_{1}<0$ falls towards a black hole while the second one with $%
E_{2}>E_{0}\,\ $moves in the outward direction. It bounces back from a
reflecting shell and, afterwards, the process repeats. For radial motion of
charged particles in the Reissner-Nordst\"{o}m metric, the result depends
strongly on a concrete scenario. In particular, an indefinitely large growth of
energy inside a shell is possible that gives rise to a black-hole bomb. We also
consider a similar multiple process with neutral particles in the background of
a rotating axially symmetric stationary black hole. We demonstrate that, if
particle decay occurs in the turning point, a black-hole bomb in this case is
impossible at all. For a generic point inside the ergoregion, there is a
condition for a black-hole bomb to exist. It relates the ratio of masses before
and after decay and the velocity of a fragment in the center of mass frame.
