Besides the force of gravity and the electromagnetic forces, there are weak and strong nuclear forces. "Weak nuclear forces" are the forces that cause radioactivity, or when matter emits energy. Weak nuclear force works on particles with a spin of 1/2. "Strong nuclear forces" are the forces that keep the quarks in a neutron and a proton together, and keeps the protons and neutrons together in an atom. The particle that carries the strong nuclear force is thought to be a gluon. The gluon is a particle with a spin of 1. The gluon holds together quarks to form protons and neutrons. However, the gluon only holds together quarks that are three different colors. This makes the end product have no color. This is called "confinement".