Neutrinos were originally thought to be massless – and possibly Weyl fermions. However, because neutrinos change flavour as they travel, at least two of the types of neutrinos must have mass (and cannot be Weyl fermions). The discovery of this phenomenon, known as neutrino oscillation, led to Canadian scientist Arthur B. McDonald and Japanese scientist Takaaki Kajita sharing the 2015 Nobel prize in physics.