To see a practical significance of this quantity let us accept as a law of physics that, if the stream of particles into S is constant and all other relevant factors are kept constant, S will eventually reach steady state (i.e. the number and distribution of particles is constant everywhere in S). It can then be demonstrated that the steady state number of particles in the subsystem s equals the stream of particles into the system S times the mean sojourn time of the subsystem. This is thus a more general form of what above was referred to as Little's theorem, and it might be called the mass-time equivalence :