Neutrinos are notorious shape-shifters. Each one is born as one of three types, or flavors -electron, muon and tau - but they can change flavor in a few thousandths of a second as they travel, as if they can't make up their mind what to be. Neutrinos, like other subatomic particles, sometimes behave like waves. But as the neutrino travels, the flavor waves combine in different ways. Sometimes the combination forms what is mostly an electron neutrino and sometimes mostly a muon neutrino. Because neutrinos are quantum particles, and by definition weird, they are not one single flavor at a time, but rather always a mixture of flavors. On the very, very rare occasion that a neutrino interacts with another particle, if the reaction appears to produce an electron, then the neutrino was an electron flavor in its final moments; if it produces a muon, the neutrino was muon-flavored. It's as if the shy neutrino's identity crisis can only be resolved when it finally interacts with another particle.
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