  Unstable particles decay sooner or later, so they are not described by
asymptotic one-particle states and they should not be included as independent
states in unitarity relations such as the optical theorem. The same applies to
any countable collection of unstable particles. We show that the behaviour of
unparticle stuff, that is, a continuous collection of particles with different
masses and common decay channels, is pretty different: it has a non-vanishing
probability of surviving for ever and the corresponding asymptotic states must
be taken into account to comply with unitarity. We also discuss compressed
spectra and the transition from the discrete to the continuous case.
