  Using the Born-Oppenheimer approximation, we show that exotic resonances, X
and Z, may emerge as QCD molecular objects made of colored two-quark lumps,
states with heavy-light diquarks spatially separated from antidiquarks. With
the same method we confirm that doubly heavy tetraquarks are stable against
strong decays. Tetraquarks described here provide a new picture of exotic
hadrons, as formed by the QCD analog of the hydrogen bond of molecular physics.
