  The confining string in presence of dynamical quarks can behave in different
ways, depending on the quark masses and on the number N_f N_c of charge
species. For light masses and large N_f N_c the creation of quark pairs
produces microscopic holes in the world- sheet swept by the string; as a
consequence string breaking is invisible in the Wilson loop, while is manifest
in operators composed of disjoint sources, as observed in many numerical
experiments. Similar arguments apply also to the adjoint string breaking. For
larger quark masses it is possible that the string world-sheet undergoes a
transition to a tearing phase, characterized by macroscopic holes, which
manifest themselves through a visible string breaking effect also in the Wilson
loop, as observed in 2+1 D SU(2) gauge theory with two flavors.
