  The customary picture of the fireball deceleration by an external medium
neglects two physical agents in the problem that are important at radii
R<10^{17} cm: a radiative precursor (the prompt GRB) and a relativistic front
of free neutrons. The radiative precursor preaccelerates the medium and loads
it with e+- pairs. This provides a new explanation for the early optical
flashes in GRBs. The front of free neutrons must form in a standard baryonic
fireball and change the mechanism of the fireball deceleration. The neutron
effect, however, disappears if the fireball is strongly dominated by the
Poynting flux. The neutrons thus provide a unique link between the progenitor
physics and the observed external blast wave. The explosion mechanism at
R<10^{17} cm is testable with upcoming observations of early afterglows.
