We show that external shocks cannot produce a variable GRB, unless they are
produced by an extremely narrow jets (angular opening of < ~10^{-4}) or if only
a small fraction of the shell emits the radiation and the process is very
inefficient. Internal shocks can produce the observed complex temporal
structure provided that the source itself is variable. In this case, the
observed temporal structure reflects the activity of the ``inner engine'' that
drives the bursts. This sets direct constraints on it.Comment: 15 page latex file with 5 PS figure. Complete uuencoded compressed PS
file is available at ftp://shemesh.fiz.huji.ac.il or at
http://shemesh.fiz.huji.ac.il/papers/SaP_aclue.u