Context: In beta-decay the kinetic energy of the electron is continuous. This led Pauli to the conclusion, that the pre-1930 picture (beta-decay = neutron -> proton + electron) is incorrect and he assumed that a third particle (the neutrino) is taking part. Question: Why would the following explanation be wrong? The spectrum of the electron's kinetic energy is continous, only because the spectrum of the proton's kinetic energy is contnious, too. The total energy kinetic energy of the neutron is then arbitrarily split between the electron and the proton. I know this explanation is wrong but how does one know, that the kinetic energy spectrum of the proton is not continuous?