When you drill down into the very fabric of reality—where elementary particles make up the matter that is you and me and everything around us in three-dimensional space—things get divided nicely into two categories: fermions and bosons. These two categories of particles are defined primarily by their atomic spin (in quantum mechanical terms), with bosons (photons, gluons, Higgs boson, W bosons, and Z bosons) having integer spin values and fermions (protons, neutrons, electrons, neutrinos) having half-integer spin values. While two bosons can occupy the same quantum state (which is why photons can pass through each other), two fermions can not—a good thing, since if they could, you’d fall through the floor right now.