  We carry out a systematic analysis of the Cabbibo-favored (CF) and singly
Cabbibo-suppressed (SCS) decays of $D^0\to VV$, and demonstrate that the
long-distance mechanism due to the final-state interactions (FSIs) can provide
a natural explanation for these mysterious polarization puzzles observed in
$D^0\to VV$ in experiments. More observables, which can be measured at BESIII
and possibly at LHCb, are also suggested.
