The quarks and gluons of the strong (or colour) force are the third piece in the puzzle of nature's four forces. The first piece, the electromagnetic force, is similar to the strong force but instead of gluons, particles of light, photons, are the force carriers. The gluons carry colour charge while the photons are electrically neutral. The second piece in the puzzle is the weak force, which controls some radioactive decays and energy production in the sun. This force differs from the other two because the force-carrying particles are very heavy. The fourth force, gravity, is the least understood even though it is experienced by us all. Gravitons are thought to be the force-carrying particles, but they have yet to be discovered. The Standard Model provides a description of all the forces apart from gravity.