  We discuss selected and important features of hadronic molecules as one of
promising forms of exotic hadrons near thresholds. Using examples of $D \bar
D^*$ systems such as $X(3872)$ and $Z_c$, emphasis is put on the roles of the
one pion exchange interaction between them and their coupling to intrinsic
quark states. Thus hadronic molecules emerge as admixtures of the dominant
long-range hadron structure and short-range quark structure. For the pion
exchange interaction, properties of the tensor force are analyzed in detail.
More coupled channels supply more attractions, and heavier constituents
suppress kinetic energies, providing more chances to form hadronic molecules of
heavy hadrons. Throughout this article, we show details of basic ideas and
methods.
