Neutrinos are devious little particles. Only in the late 1990s were they shown to have mass, after decades of head-scratching hints to that effect. They can oscillate between three neutrino types, or "flavors," changing their identity on the fly. And, perhaps most famously, they were accused just last year of breaking cosmic law by traveling faster than light. (The jury is out, but an acquittal appears imminent .)