  Extended emission is a mystery in short gamma-ray bursts (SGRBs). By making
time resolved spectral analyses of brightest nine events observed by ${\it
Swift}$ XRT, we obviously classify the early X-ray emission of SGRBs into two
types. One is the extended emission with exponentially rapid decay, which shows
significant spectral softening during hundreds seconds since the SGRB trigger
and is also detected by ${\it Swift}$-BAT. The other is a dim afterglow only
showing power-law decay over $10^4$ s. The correlations between the temporal
decay and spectral indices of the extended emissions are inconsistent with the
$\alpha$-$\beta$ correlation expected for the high-latitude curvature emission
from a uniform jet. The observed too-rapid decay suggests the emission from a
photosphere or a patchy surface, and manifests the stopping central engine via
such as magnetic reconnection at the black hole.
