  We propose that the flat decay phase in the first 100-10,000 seconds of the
X-ray light curve of Gamma Ray Bursts can be interpreted as prolonged activity
of the central engine, producing shells of decreasing bulk Lorentz factors
Gamma. The internal dissipation of these late shells produces a continuous and
smooth emission, usually dominant in X-rays and sometimes in the optical. When
Gamma of the late shells is larger than 1/theta_j, where theta_j is the jet
opening angle, we see only a portion of the emitting surface. Eventually, Gamma
becomes smaller than 1/theta_j, and the entire emitting surface is visible.
When Gamma=1/theta_j there is a break in the light curve, and the plateau ends.
During the plateau phase, we see the sum of the "late-prompt" emission (due to
late internal dissipation), and the "real afterglow" emission (due to external
shocks). A variety of different optical and X-ray light curves is possible,
explaining why the X-ray and the optical light curves often do not track each
other, and why they often do not have simultaneous breaks.
