In the summer of '53, after I had packed up the furniture and left Cornell, and before arriving at Princeton, we had three months in Berkeley where I was invited by Charles Kittel, who was professor of solid state physics, to work on problems which he supplied, and he was a wonderful supplier of problems, like Hans Bethe; he gave me things to think about which were just suited to my abilities. They were problems of a well-defined nature where many particle systems could be handled by the techniques that I had developed for quantum electrodynamics. So it was the first time that we applied what later came to be called the Green Function methods in solid state physics. And I did first of all a problem of a ferromagnetic resonance in metals, of spin resonance in metals, and then the next time I was there, two years later, I did a major study of spin waves, which was one of the main things that I am proud of, this general theory of spin waves. In fact it was a very beautiful piece of mathematics, everything worked just marvellously, just as beautiful as quantum electrodynamics - not as important but still, from my point of view, just as satisfying. So I was able to deal with spin waves in a very elegant and systematic way.