Experiments at high energies showed that quarks which protons and neutrons
consist of, are several hundred times lighter than the proton, in accordance
with the fundamental theory of Quantum Chromodynamics (QCD). On the other
hand, at low energies protons and neutrons, as well as other hadrons, behave
as if they are built of very different quarks - the so-called constituent
quarks with masses of the order of 1/3 of the proton mass. That is, they seem
to be some hundred times more massive than the 'fundamental' quarks explaining
the experiments in high-energy particle physics. The resolution of this puzzle
is that at low energies, QCD is strongly interacting and generates dressed,
effectively massive quarks through a nonperturbative effect called dynamical
chiral symmetry breaking.