  After examining the Feynman diagrams corresponding to the $\bar D^{(*)}
\Sigma_c^{(*)}$, $\bar D^{(*)} \Lambda_c$, $D^{(*)} \bar K^{*}$, and $D^{(*)}
\bar D^{(*)}$ hadronic molecular states, we propose a possible binding
mechanism induced by shared light quarks. This mechanism is similar to the
covalent bond in chemical molecules induced by shared electrons. We use the
method of QCD sum rules to calculate its corresponding light-quark-exchange
diagrams, and the obtained results indicate a model-independent hypothesis: the
light-quark-exchange interaction is attractive when the shared light quarks are
totally antisymmetric so that obey the Pauli principle. We build a toy model
with four parameters to formulize this picture, and estimate binding energies
of some possibly-existing covalent hadronic molecules. A unique feature of this
picture is that binding energies of the $(I)J^P = (0)1^+$ $D\bar B^*/D^* \bar
B$ hadronic molecules are much larger than those of the $(I)J^P = (0)1^+$
$DD^*/\bar B \bar B^*$ ones, while the $(I)J^P = (1/2)1/2^+$ $\bar D
\Sigma_c/\bar D \Sigma_b/B \Sigma_c/B \Sigma_b$ hadronic molecules have similar
binding energies.
