  We discuss the origin of X-Ray Flashes (XRFs), a recently discovered class of
Gamma-Ray Bursts (GRBs). Using a simplified model for internal shocks we check
if XRFs can be intrinsically soft due to some specific values of the parameters
describing the relativistic outflow emerging from the central engine. We
generate a large number of synthetic events and find that XRFs are obtained
when the contrast Gamma_max/Gamma_min of the Lorentz factor distribution is
small while the average Lorentz factor Gamma is large. A few XRFs may be GRBs
at large redshifts but we exclude this possibility for the bulk of the
population. If outflows with a small contrast are commonly produced, even a
large population of XRFs could be explained. If conversely the Lorentz factor
distribution within the wind is broad, one should then rely on extrinsic
causes, such as viewing angle effects or high redshift.
