Neutrons are subatomic particles that, along with protons, make up the nucleus of every atom except for hydrogen. They have no electric charge and are neutral. Neutrons are slightly more massive than positively charged protons. They are composite particles made up of three smaller, elementary particles called quarks, held together by the Strong Force. Specifically, a neutron contains one up and two down quarks. Neutrons are important tools for research in medicine, materials, and other fields. Scientists produce neutrons at research reactors and particle accelerators and project them onto samples of materials. Some of the neutrons interact directly with atoms in the sample and “bounce” away at different angles, like cue balls colliding in a game of pool. This technique is called neutron scattering. Neutrons play a major role in the mass and radioactive properties of atoms. They are bound together with other neutrons and protons in the atomic nucleus. A free neutron, one that is not incorporated into a nucleus, is subject to radioactive decay of a type called beta decay. It breaks down into a proton, an electron, and an antineutrino.