“Oscillations” refer to the way neutrinos change flavor as they travel. A neutrino born as one flavor (electron, muon, or tau neutrino) will eventually morph into the other varieties—and the probability of appearing as a different flavor depends on how far it has gone. Oscillations are a result of quantum mechanics, or the weird way things work at very small scales. Discovering neutrino oscillations was particularly exciting because it revealed to scientists that neutrinos have mass, something the current model couldn’t explain. It’s the first and only evidence that the current model of particle physics may not be complete. That means more fun physics ahead to explain this massive mystery.