In the modern physics, four fundamental interactions are known to exist in the Universe, namely, strong, electromagnetic, weak, and gravitational interactions. Among them, the strong interaction governs the behavior of atomic nuclei and their constituents, quarks and gluons. One astonishing feature of the strong interaction is that, at low energies (or low temperatures below 1 trillion degree), the interaction is so strong that the vacuum itself is not a trivial issue at all; you may think the vacuum is nothing, but it is not true. 
Such non-triviality is also reflected in the properties of strongly interacting particles, called hadrons. As an example, I will take proton, the nucleus of a hydrogen atom. Proton is quite familiar to us, being a fundamental constituent of our Universe, but still there are mysteries regarding its structure. One of such mysteries is how you understand the proton spin, 1/2, from its constituents. I will introduce some basic features of proton structure and discuss the proton spin puzzle based on the present viewpoints. Then I will show how we are attacking the problem experimentally.