  We investigate the ratio of gravitational binding energy to rest mass in
general relativity. For N pointlike masses, an upper bound on the magnitude of
this ratio can be derived using the second law of black hole dynamics. Only as
N approaches infinity can it approach one. A configuration that saturates the
$N = \infty$ bound is a thin spherical shell as its radius is taken to zero.
This system provides, in principle, a perfectly efficient mechanism for
converting rest mass into energy. We mention possible implications for the
black hole information problem.
