The neutrino is an elementary particle. It has half-integer spin and is therefore a fermion. All neutrinos observed to date have left-handed chirality. Although they had been considered massless for many years, recent experiments have shown their mass to be non-zero. Because it is an electrically neutral lepton, the neutrino interacts neither by way of the strong nor the electromagnetic force, but only through the weak force and gravity. Because the cross section in weak nuclear interactions is very small, neutrinos can pass through matter almost unhindered. For typical neutrinos produced in the sun (with energies of a few MeV), it would take approximately one light year of lead to block half of them. Detection of neutrinos is therefore challenging, requiring large detection volumes or high intensity artificial neutrino beams. There are three known types (flavors) of neutrinos: electron neutrino, muon neutrino and tau neutrino, named after their partner leptons in the Standard Model.