When, as a newcomer to the profession, I started to teach elementary Mechanics, I taught it more or less as I had been taught myself. It was a classroom subject, one which drew on the experience of the pupil certainly, but one in which actual experiment was never attempted. The ideas seemed to me to be fundamentally simple, and to be so interlocked and so few in number that it ought to be possible to communicate them to a class in a very short time. I need hardly say that I was rapidly disillusioned. I began to be faced with “Why?” questions that I could not answer convincingly, and with questions like “What is the difference between momentum and kinetic energy?” and “Is it sine or cosine?” which revealed a complete failure to understand. I found boys dividing their answers by thirty-two, because they were confused by my somewhat haphazard use of gravitational and absolute units. One boy made it plain to me that he believed that my Mechanics had no practical bearing on the world in which he lived. Much later another boy said that Mechanics to the mathematician was substituting in the right formula. I realised that some fundamental re-thinking was called for.