  Particle decays do not constitute a spin "measurement" in the
quantum-mechanical sense, but still modify the spin state, in particular for an
entangled system. We show that for a spin-entangled pair of particles the
entanglement of the system can increase after the decay of one particle. This
unique phenomenon has no equivalent for stable particles and could be
observable in top pair production at a high-energy polarized $e^+ e^-$
collider.
