The quarks constituting protons and neutrons are held together by the strong nuclear force: gluon particles (the messenger particles of the strong force) stream between quarks, “gluing” them together. Experiments have shown that these gluons are highly energetic, and since Einstein’s E=mc2 tells us that energy (E) can manifest itself as mass (m), we learn that the gluons inside protons and neutrons contribute a significant fraction of these particles’ total mass.
