The strong nuclear force is responsible for binding quarks, the fundamental particles that make up protons and neutrons, together. The strong nuclear force is propagated through a particle known as a gluon. Gluons are incredibly short-lived particles that have a minuscule interaction distance, meaning that particles must be extremely close together to allow interaction through the strong nuclear force. For lighter elements, the act of nuclear binding through the strong nuclear force releases energy in a process we know as nuclear fusion. Heavier elements, such as iron, take more energy to fuse than they release through this process.