A study published recently in the American Physical Society journal Physical Review C reveals new findings about the strong nuclear force, the mysterious fundamental force that holds together the protons and neutrons of the atomic nucleus. Experiments conducted at Argonne National Laboratory have shown how the round, heavy nuclei of the nickel-64 isotope (containing 28 protons and 36 neutrons, making it the heaviest stable Ni isotope) changed into one of two shapes—either like a doorknob or a football—depending on the amount of energy exerted on it. A summary of the research on the Phys.org website compares the nuclei shape change to popcorn kernels changing shape when heated in a microwave.