Niels Bohr and John Archibald Wheeler applied the liquid drop model of the atomic nucleus to explain the mechanism of nuclear fission. As the experimental physicists studied fission, they uncovered puzzling results. George Placzek asked Bohr why uranium seemed to fission with both fast and slow neutrons. Walking to a meeting with Wheeler, Bohr had an insight that the fission at low energies was due to the uranium-235 isotope, while at high energies it was mainly due to the far more abundant uranium-238 isotope. The former makes up just 0.714 percent of the uranium atoms in natural uranium, about one in every 140; natural uranium is 99.28 percent uranium-238. There is also a tiny amount of uranium-234, which accounts for just 0.006 percent.