'Can you tell us how a bomb works, sir?'

'One day,' I said.

'A nuclear bomb?'

'One day.'

'A hydrogen… cobalt… neutron bomb?'

'One day.'

They never asked how radio waves crossed the ether, which was to me a greater mystery. They asked about destruction, not creation; about power, not symmetry. The seed of violence born in every male child looked out of every face, and I knew how they were thinking, because I'd been there myself. Why else had I spent countless hours at their age practising with a. 22 cadet rifle on a range, improving my skill until I could hit a target the size of a thumbnail at fifty yards, nine times out of ten. A strange, pointless, sublimated skill, which I never intended to use on any living creature, but had never since lost.

'Is it true, sir,' one of them said,'that you won an Olympic medal for rifle shooting?'

'No, it isn't.'

'What, then, sir?'

'I want you all to consider the speed of a bullet compared to the speed of other objects you are all familiar with. Now, do you think that you could be flying along in an aeroplane, and look out of the window, and see a bullet keeping pace with you, appearing to be standing still just outside the window?'

The lesson wound on. They would remember it all their lives, because of the gun. Without the gun, whatever Jenkins might think, it would have faded into the general dust they shook from their shoes every afternoon at four o'clock. Teaching, it often seemed to me, was as much a matter of image-jerking as of imparting actual information. The facts dressed up in jokes were the ones they got right in exams.

I liked teaching. Specifically I liked teaching physics, a subject I suppose I embraced with passion and joy, knowing full well that most people shied away in horror. Physics was only the science of the unseen world, as geography was of the seen. Physics was the science of all the tremendously powerful invisibilities – of magnetism, electricity, gravity, light, sound, cosmic rays… Physics was the science of the mysteries of the universe. How could anyone think it dull?