  Model-building issues raised by the prospect of light sterile neutrinos are
discussed in a pedagogical way. I first review the na\"{\i}ve proposal that
sterile neutrinos be identified with ``right handed neutrinos''. A critical
discussion of the simple expedient of adding three gauge singlet fermions to
the usual minimal standard model matter content is followed by an examination
of right handed neutrinos in extended theories. I introduce the terminology of
``fully sterile'' and ``weakly sterile'' to classify varieties usually
conflated under the sterile neutrino banner. After introducing the concepts of
``technical naturalness'' and plain ``naturalness'', the unbearable lightness
of being a sterile neutrino is confronted. This problem is used to motivate
mirror neutrinos, whose connection with pairwise maximal mixing is emphasised.
Some brief remarks about phenomenology are made throughout. The impossibility
of identifying the sole sterile neutrino of the currently favoured $2 + 2$ and
$3 + 1$ phenomenological constructs as a lone gauge singlet fermion added to
the minimal standard model is explained. Finally, I remark on the beauty and
subtlety of light sterile neutrino cosmology.
