  The relativistic jets of SS433 are outstanding for their optical thermal
radiation.The radiation is produced by small clouds ($10^8$ cm) whose lifetime
is about $10^3$ times larger than the gas-dynamical crushing time. We show that
the clouds reside in thermal and dynamical balance as long as they
collisionally interact with the wind of the supercritical accretion disk. The
interaction is caused by the precessional movement of the jets and takes place
only in the sweep-out zone. Beyond the sweep-out zone the interaction ceases
and optical jets just terminate. The cloud magnetic field amplified in course
of movement through a medium could play a role in containing a cloud. Thus, the
clue to the uniqueness of the optical jets of SS433 is thought to be their
precessional movement, which provides an opportunity for collisional
interaction of the clouds with the wind.
