  Recently the AGILE and Fermi/LAT detectors uncovered giant $\gamma$-ray
flares from the Crab nebula. The duration of these flares is a few days. The
Fermi/LAT data with monthly time binning further showed significant variability
of the synchrotron tail of the emission, while the inverse Compton component
was stable. The simultaneous or follow-up observations in X-ray, optical,
infrared and radio bands did not find significant flux variation. Based on
these observations, we propose that the $\gamma$-ray variability and flares are
due to statistical fluctuations of knots that can accelerate electrons to
$\sim$PeV energies. The maximum achievable energy of electrons is adopted to be
proportional to the size of the knot, which is assumed to follow a power-law
distribution. Thus the low energy electron flux will be stable due to the large
number of small knots, while the high energy electron flux may experience large
fluctuations. Monte Carlo realization of such a picture can reproduce the
observational data quite well given proper model parameters.
