The model of fission we had, I can compare to nothing better than the picture of a hill, and you're skating back and forth, and that corresponds to the vibration of an atomic nucleus. But if you have enough energy in your skating, you could go over the hill and go down, and that corresponds to the nucleus stretching so far that it falls apart and explodes. If you can get the 5 million electron volts of energy, enough to get over the hill, then you'll get 200 million electron volts of energy as you go down the hill. That's nuclear fission, in brief. That was our picture of it. But that picture said the whole thing that governed fission was the height of this hill you had to get over to split. You would call it the Fission Barrier. It's a barrier you have to surmount to have split. And this fission barrier, we had to conclude, would be lower in an atomic nuclei with odd numbers of particles than in similar nuclei with even numbers of particles. Therefore, to be lower in Uranium 235 than in Uranium 238. Uranium 235 is the rare constituent of uranium. And we concluded then that it's the one that can be split by lower energy neutrons, whereas the uranium 238 requires higher energy bombardment to split.