  Freak waves, or rogue waves, are one of the fascinating manifestations of the
strength of nature. These devastating walls of water appear from nowhere, are
short-lived and extremely rare. Despite the large amount of research activities
on this subject, neither the minimum ingredients required for their generation
nor the mechanisms explaining their formation have been given. Today, it is
possible to reproduce such kind of waves in optical fibre systems. In this
context, we demonstrate theoretically and numerically that convective
instability is the basic ingredient for the formation of rogue waves. This
explains why rogues waves are extremely sensitive to noisy environments.
