I had, in that connection myself, something of a difficulty. The whole work seemed to be planned by Johnny von Neumann. Well, he told us about methods to produce in high explosives a spherical convergent motion. But the question, the hydrodynamics of an imploding shell, required a solution of differential equations that looked to me anything but simple. The Head of the Physics Department, of the theoretical work, Hans Bethe, wanted me to do this calculation. I begged off. It looked to me impossible to form- perform in an accurate manner. And there was a way out, because by that time, computers had made a lot of progress and instead of accurate analytic solutions, we could work with computer solutions, with numerical solutions. That would not have been, in itself, so hard. The difficulty was that the whole situation was a kind of motion which we call unstable. If we have a little inaccuracy in it, that the sphere was not quite a sphere, a shell was not a uniform shell but at some place it was a little thinner than at other places, then this lack of symmetry will grow. And to avoid that and to produce something really reliable, seemed to me a very difficult problem. I begged off. I don't think Bethe ever forgave me. Oppenheimer apparently, I think, in fact, understood. He let me do things that I felt I could accomplish better.