I'm one of these poor devils who can't understand anything unless they can draw a picture. And so I wanted a picture showing the height of the barrier that'd have to be climbed over to get fission as it depended on the number of particles in the nucleus. And from that it became clear that the barrier would be lower for uranium 235 than for uranium 238. And just at this time, the measurements came in showing you would get fission for low energy neutrons, and you'd get fission for high energy neutrons, but not very much for neutrons of intermediate energy. At breakfast, at the Nassau Club, where Niels Bohr was living, in Princeton, Placzek, a visitor, said- Your Theory of Fission with Wheeler is cuckoo, because you find this strange behavior. You can't understand it. And Bohr, walking back from the Nassau Club to Fine Hall where we had our offices, thinking about this, and he said- the answer is that it's U235 for the slow neutrons, and U238 for the fast neutrons. And when he got to the office, we drew the diagrams on the board, and I think Placzek ended up almost convinced, but nevertheless, he bet that I was wrong. And I bet him, to show my faith that we were right, I bet him the ratio of the mass of the proton to the mass of the electron, $18.35, compared to one penny. So if he lost, he would pay me one penny, but if I lost, I'd pay him $18.35. Well, the experiments were not fully available, on separating uranium 235 and separating uranium 238, at that time. But one year later there were experiments and I got a telegram in the mail, a Western Union Telegram Postal Money Order for one penny, with the word 'congratulations' from Placzek. But anyway, that was the route to conviction that uranium 235 is a special material. But to separate it from uranium 238, for making a practical use of it, would be a terrible job, because they're chemically similar. But one of the things on this great chart that I speak about, that I insisted on drawing, was that there were other nuclei. And two stages beyond uranium was a nucleus that had not been named because it hadn't yet been discovered, but it had to exist, according to us, [U]239. And that would also be fissile, would split easily with neutrons. And not long later, Glenn Seaborg, at Berkeley, was making this element, and determining its chemistry. And the great thing about plutonium is that it's chemically different from uranium so that it can be separated from it if you could make it.