Neutrino
The three types of neutrinos change into each other over time, so an electron neutrino could turn into a muon or tau neutrino and then back again. This is called neutrino oscillation. This oscillation was suggested when then number of electron neutrinos from the Sun was measured in the 1960s, and the amount was only about a third of what theories at the time said there should be. The result of neutrino oscillation is that most of the electron neutrinos made in the Sun change to another type of neutrino .