In particle physics, the strong interaction is the mechanism responsible for the strong nuclear force (also called the strong force, nuclear strong force), one of the four known fundamental interactions of nature, the others being electromagnetism, the weak interaction and gravitation. Despite only operating at a distance of a femtometer, it is the strongest force, being approximately 100 times stronger than electromagnetism, a million times stronger than weak interaction and 10³⁸ times stronger than gravitation at that range. It ensures the stability of ordinary matter, confining quarks into hadron particles, such as the proton and neutron, the largest components of the mass of ordinary matter. Furthermore, most of the mass-energy of a common proton or neutron is in the form of the strong force field energy; the individual quarks provide only about 1% of the mass-energy of a proton.