Quantum chromodynamics (QCD) is granted to be complete and thus explain all the spectra, structures and reactions of hadrons. Yet, in reality, we have much difficulty in explaining the observed hadron spectra, their structures or production mechanisms. The "quark model" is a well working model, while its foundation from QCD is not established. It is mysterious why hadrons beyond the quark model are rare (or none). I would like to discuss possible multi-quark systems with heavy quark(s) and their implications in QCD.