Leptons do not interact via the strong interaction. Their respective antiparticles are the antileptons, which are identical, except that they carry the opposite electric charge and lepton number. The antiparticle of an electron is an antielectron, which is almost always called a " positron " for historical reasons. There are six leptons in total; the three charged leptons are called "electron-like leptons", while the neutral leptons are called " neutrinos ". Neutrinos are known to oscillate, so that neutrinos of definite flavor do not have definite mass: Instead, they exist in a superposition of mass eigenstates. The hypothetical heavy right-handed neutrino, called a " sterile neutrino ", has been omitted.