  We consider, in general terms, the early thermal evolution of an isolated
neutron star, i.e., during the first $10^5$ years after the supernova explosion
when the cooling is driven by neutrino emission from the core. It is shown
that, if fast neutrino emission is occuring, the surface temperature is
actually determined by the critical temperature $T_c$ at which the core baryons
become superfluid, and almost nothing else. In particular, the dependence on
the actual neutrino emission process is very weak. In other terms, fast
neutrino cooling neutron stars are thermometers for the highest $T_c$
superfluid (superconductor) in the Universe. The observational data are briefly
presented.
