Quarks are fundamental particles that respond to the strong force. The strong force is one of the four fundamental forces, and dominates in strength over the other three forces—electromagnetic, weak, and gravity—at very short distances (of the order of 10⁻¹⁵ m, the size of an atomic nucleus). Quarks do not appear in isolation but are confined in clusters known as hadrons. The nucleus of the hydrogen atom, the proton, is the most familiar example of such a hadron. The proton requires three quarks to combine to account for its overall electric charge and other properties. The family of such triplets is known as baryons.