'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.