  The Gamma Ray Burst GRB920229 has four extreme and unprecedented properties;
a rise in brightness with an e-folding time scale of $220 \pm 30 \mu s$, a fall
in brightness with an e-folding time scale of $400 \pm 100 \mu s$, a large
change in spectral shape over a time of $768 \mu s$, and a sharp spectral
cutoff to high energies with $\Delta E/E = 18 %$. The rapid changes occur
during a spike in the light curve which was seen 0.164 s after the start of the
burst. The spectrum has a peak $\nu F_{\nu}$ at 200 keV with no significant
flux above 239 keV, although the cutoff energy shifts to less than 100 keV
during the decay of the spike. These numbers can be used to place severe limits
on fireball models of bursts. The thickness of the energy production region
must be smaller than $\sim 66 km$, ejected shells must have a dispersion of the
Lorentz factor of less than roughly 1% along a particular radius, and the
angular size of the radiation emission region is of order 1 arc-minute as
viewed from the burst center. The physical mechanism that caused the sharp
spectral cutoff has not been determined.
