I don't really understand why neutrino oscillation occurs so I looked it up on wikipedia: http://en.wikipedia.org/wiki/Neutrino_oscillation I went to the "Theory, graphically" section and it gives quite a good explanation. However, there are a few details I don't get. It says: "This flavor state is a combination of mass states" - How can that be? and "However, each mass state is also made up of flavor states" - How can that be? In short, it may be due to my lack of understand about flavour (I always assumed it was simply the name given to particles which are the same except in mass), but I don't understand how a mass state can be made up of a flavour state or vice versa. Can anyone help?