Neutrino Oscillation
A neutrino produced by the decay of another particle has a well defined flavour (electron, muon, or tau). However, if the neutrino propagates and the flavour is measured after a certain time, a different result may be observed. Quantum mechanics explains this startling effect: a neutrino is considered as the superposition of three waves which correspond to particles of different masses. The latter do not propagate at the same frequency, which alters the blend composition over time.