  We present the observations of GRB090510 performed by the Fermi Gamma-Ray
Space Telescope and the Swift observatory. This is a bright, short burst that
shows an extended emission detected in the GeV range. Furthermore, its optical
emission initially rises, a feature so far observed only in long bursts, while
the X-ray flux shows an initial shallow decrease, followed by a steeper decay.
This exceptional behavior enables us to investigate the physical properties of
the GRB outflow, poorly known in short bursts. We discuss internal shock and
external shock models for the broadband energy emission of this object.
