I told the boys to stay quiet while I went to fetch my gun.

It usually worked. For the five minutes that it took me to get to the locker in the common-room and to return to the classroom, thirty fourteen-year-old semi-repressed hooligans could be counted on to be held in a state of fragile good behaviour, restrained only by the promise of a lesson they'd actually looked forward to. Physics in general they took to be unacceptably hard mental labour, but what happened when a gun spat out a bullet… that was interesting.

Jenkins delayed me for a moment in the common-room: Jenkins with his sour expression and bad-tempered moustache, telling me I could teach momentum more clearly with chalk on a blackboard, and that an actual firearm was on my part simply self-indulgent dramatics.

'No doubt you're right,' I said blandly, edging round him.

He gave me his usual look of frustrated spite. He hated my policy of always agreeing with him, which was, of course, why I did it.

'Excuse me,' I said, retreating, 'Four A are waiting.'

Four A, however, weren't waiting in the hoped-for state of gently simmering excitement. They were, instead, in collective giggles fast approaching mild hysteria.

'Look,' I said flatly, sensing the atmosphere with one foot through the door, 'steady down, or you'll copy notes…'

This direst of threats had no result. The giggles couldn't be stifled. The eyes of the class darted between me and my gun and the blackboard, which was still out of my sight behind the open door, and upon every young face there was the most gleeful anticipation.

'OK,' I said, closing the door, 'so what have you writ-'

I stopped.

They hadn't written anything.

One of the boys stood there, in front of the blackboard, straight and still: Paul Arcady, the wit of the class. He stood straight and still because, balanced on his head, there was an apple.

The giggles all around me exploded into laughter, and I couldn't myself keep a straight face.

'Can you shoot it off, sir?'

The voices rose above a general hubbub.

'William Tell could, sir.'

'Shall we call an ambulance, sir, just in case?'

'How long will it take a bullet to get through Paul's skull, sir?'

'Very funny,' I said repressively, but indeed it was very funny, and they knew it. But if I laughed too much I'd lose control of them, and control of such a volatile mass was always precarious.

'Very clever, Paul,' I said. 'Go and sit down.'

He was satisfied. He'd produced his effect perfectly. He took the apple off his head with a natural elegance and returned in good order to his place, accepting as his due the admiring jokes and the envious catcalls.

'Right then,' I said, planting myself firmly where he had stood, 'by the end of this lesson you'll all know how long it would take for a bullet travelling at a certain speed to cross a certain distance…'

The gun I had taken to the lesson had been a simple air-gun, but I told them also how a rifle worked, and why in each case a bullet or a pellet came out fast. I let them handle the smooth metal: the first time many of them had seen an actual gun, even an air-gun, at close quarters. I explained how bullets were made, and how they differed from the pellets I had with me. How loading mechanisms worked. How the grooves inside a rifle barrel rotated the bullet, to send it out spinning. I told them about air friction, and heat.

They listened with concentration and asked the questions they always did.

'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?

I had been for three years head of the physics department of the East Middlesex Comprehensive, with four masters and two technicians within my domain. My future, from my present age of thirty-three, looked like a possible deputy head-mastership, most likely with a move involved, and even perhaps a headship, though if I hadn't achieved that by forty I could forget it. Headmasters got younger every year; mostly, cynics suggested, because the younger the man they appointed, the more the authorities could boss him about.

I was, all in all, contented with my job and hopeful of my prospects. It was only at home that things weren't so good.

Four A learned about momentum and Arcady ate his apple when he thought I wasn't looking. My peripheral vision after ten years of teaching was, however, so acute that at times they thought I could literally see out of the back of my head. It did no harm: it made control easier.

'Don't drop the core on the floor, Paul,' I said mildly. It was one thing to let him eat the apple – he'd deserved it – but quite another to let him think I hadn't seen. Keeping a grip on the monsters was a perpetual psychological game, but also priority number one. I'd seen stronger men than myself reduced to nervous breakdowns by the hunting-pack instincts of children.

When the end-of-lesson bell rang, they did me the ultimate courtesy of letting me finish what I was saying before erupting into the going-home stampede. It was, after all, the last lesson on Friday – and God be thanked for weekends.

I made my way slowly round the four physics laboratories and the two equipment rooms, checking that everything was in order. The two technicians, Louisa and David, were dismantling and putting away all apparatus not needed on Monday, picking Five E's efforts at radio-circuitry to pieces and returning the batteries, clips, bases and transistors to the countless racks and drawers in the equipment rooms.