  We explore two possible models which might give rise to bright X-ray flares
in GRBs afterglows. One is an external forward-reverse shock model, in which
the shock parameters of forward/reverse shocks are taken to be quite different.
The other is a so called "late internal shock model", which requires a
refreshed unsteady relativistic outflow generated after the prompt $\gamma-$ray
emission. In the forward-reverse shock model, after the time $t_\times$ at
which the RS crosses the ejecta, the flux declines more slowly than
$(t_\oplus/t_\times)^{-(2+\beta)}$, where $t_\oplus$ denotes the observer's
time and $\beta$ is the spectral index of the X-ray emission. In the ``late
internal shock model", decaying slopes much steeper than $(t_\oplus/t_{\rm e,
\oplus})^{-(2+\beta)}$ are possible if the central engine shuts down after
$t_{\rm e, \oplus}$ and the observed variability timescale of the X-ray flare
is much shorter than $t_{\rm e, \oplus}$.
  The sharp decline of the X-ray flares detected in GRB 011121, XRF 050406, GRB
050502b, and GRB 050730 rules out the external forward-reverse shock model
directly and favors the "late internal shock model". These X-ray flares could
thus hint that the central engine operates again and a new unsteady
relativistic outflow is generated just a few minutes after the intrinsic hard
burst.
