Neutrinos

A neutrino (Italian pronunciation: [neuˈtriːno], meaning "small neutral one"; English pronunciation: /njuːˈtriːnoʊ/) is an elementary particle that usually travels close to the speed of light, is electrically neutral, and is able to pass through ordinary matter almost undisturbed. This makes neutrinos extremely difficult to detect. Neutrinos have a very small, but nonzero mass. They are denoted by the Greek letter ν (nu).
Neutrinos are created as a result of certain types of radioactive decay or nuclear reactions such as those that take place in the Sun, in nuclear reactors, or when cosmic rays hit atoms. There are three types, or "flavours", of neutrinos: electron neutrinos, muon neutrinos and tau neutrinos; each type also has a corresponding antiparticle, called antineutrinos. Electron neutrinos (or antineutrinos) are generated whenever protons change into neutrons (or vice versa), the two forms of beta decay. Interactions involving neutrinos are mediated by the weak interaction.
Most neutrinos passing through the Earth emanate from the Sun, and more than 50 trillion solar neutrinos pass through an average human body every second.