  When a star passes through the tidal disruption radius of a massive black
hole (BH), it can be torn apart by the tidal force of the BH, known as the
Tidal Disruption Event (TDE). Since the UV/optical emitting region inferred
from the blackbody radius is significantly larger than the circularization
radius predicted by the classical TDE theory, two competing models, stream
collision and envelope reprocessing, were proposed to explain the unexpectedly
large UV/optical emitting size. Here, we investigate the variability behaviors
(cross-correlation and time delay) of three representative TDEs with continuum
reverberation mapping. Our results demonstrate that TDE behavior is clearly
inconsistent with the envelope reprocessing scenario. In contrast, the picture
of the stream collision, together with the late-time formed accretion disk, can
explain heterogeneous observations. This provides compelling evidence that the
UV/optical emission originates from stream collisions during the early-stage of
TDE evolution and gradually transitions to being dominated by accretion disk
with detectable X-ray emission in a late stage. After fading back to a
quiescent state, recurrent flares may be observed in some occasions, such as
partial TDEs.
