"Neutrons can dilute the repulsion, while the strong nuclear force can overcome that repulsion to force protons into proximity. According to Sam Kean, the strong force is like the T.Rex because it is strong but has short arms. That is, the strength of the interaction drops off fast with increasing distance, so that as a nucleus swells, the strong force is less able to hold particles together, and so the nucleus splits in a process called fission or emits particles in some kind of decay. "