  Three relativistic particles in addition to the photon are detected in the
cosmic microwave background (CMB). In the standard model of cosmology, these
are interpreted as the three neutrino species. However, at the time of
CMB-decoupling, neutrinos are not only relativistic but they are also
freestreaming. Here, we investigate, whether the CMB is sensitive to this
defining feature of neutrinos, or whether the CMB-data allow to replace
neutrinos with a relativistic fluid. We show that free streaming particles are
preferred over a relativistic perfect fluid with $\Delta\chi^2\simeq 21$. We
also study the possibility to replace the neutrinos by a viscous fluid and find
that a relativistic viscous fluid with either the standard values $c_{\rm
eff}^2=c_{\rm vis}^2=1/3$ or best fit values for $c_{\rm eff}^2$ and $c_{\rm
vis}^2$ has $\Delta\chi^2=20$ and thus cannot provide a good fit to present CMB
data either.
