  Several conserved and/or gauge invariant quantities described as the
second-order curvature perturbation have been given in the literature. We
revisit various scenarios for the generation of second-order non-gaussianity in
the primordial curvature perturbation \zeta, employing for the first time a
unified notation and focusing on the normalisation f_{NL} of the bispectrum.
When the classical curvature perturbation first appears a few Hubble times
after horizon exit, |f_{NL}| is much less than 1 and is, therefore, negligible.
Thereafter \zeta (and hence f_{NL}) is conserved as long as the pressure is a
unique function of energy density (adiabatic pressure). Non-adiabatic pressure
comes presumably only from the effect of fields, other than the one pointing
along the inflationary trajectory, which are light during inflation (`light
non-inflaton fields'). During single-component inflation f_{NL} is constant,
but multi-component inflation might generate |f_{NL}| \sim 1 or bigger.
Preheating can affect f_{NL} only in atypical scenarios where it involves light
non-inflaton fields. The curvaton scenario typically gives f_{NL} \ll -1 or
f_{NL} = +5/4. The inhomogeneous reheating scenario can give a wide range of
values for f_{NL}. Unless there is a detection, observation can eventually
provide a limit |f_{NL}| \lsim 1, at which level it will be crucial to
calculate the precise observational limit using second order theory.
