I wish I could say that the class was held enthralled by this classic old-fashioned piece of storytelling. Maybe there was just a bit less wailing, nose-picking, staring at the ceiling, or nudging than usual. But what mainly emerged as I asked them about the story afterward was that nobody knew what a watermelon was. I drew one on the blackboard for them with red and green chalk. A watermelon is so like a cartoon anyway that even I can draw them. A complete blank.

So I said that if they were good-and for the last hour of the afternoon they were alarmingly well behaved-I’d bring in a watermelon for them the next day. On the way home I got off the bus a stop later than usual, after it had turned up Seven Sisters Road. I walked back down the road past the greengrocers and stalls. In the very first one I bought a pound of golden nap cherries and ate them greedily. They were tart, juicy, clean; they made me think of being in the countryside where I grew up, of sitting under the green shade as the sun goes down. It was just after five o’clock, so the traffic was already starting to grind to a halt. The fumes were hot against my face, but I was feeling almost cheerful. I was fighting my way through crowds of people as usual, but many of them seemed in good spirits. They were wearing bright colors. My urban claustrophobia meter was down from its usual eleven to a more manageable six or seven or so.

I bought a watermelon the size of a basketball and the weight of a bowling ball. The man needed four carrier bags one inside the other, and there was virtually no practical way of carrying it. Very gingerly I swung the bag over my shoulder, almost spinning myself into the traffic as I did so, and carried the melon like a man with a sack of coal on his back. It was only about three hundred yards to the flat. I’d probably make it.

As I crossed Seven Sisters Road and turned into Holloway Road, people stared at me. God knows what they thought I was up to, a skimpily dressed young blonde hunched over and carrying what must have looked like her own weight in iron ore in a shopping bag.

Then it happened. What did it feel like at the time? It was a moment, an impulse, a blow, and then it was in the past. I only really reconstructed what had taken place through the action replays in my mind, by telling people about it, by what people told me about it. A bus was coming toward me on the inside lane of the road. It had almost reached me when a person jumped off the platform at the back. The bus was going as close to full speed as anything ever gets on Holloway Road during rush hour. Normal people don’t jump off buses like that, even Londoners, so at first I thought he may have been recklessly crossing the road behind the bus. It was the speed at which he hit the pavement, almost losing his balance, that showed he must have come off the bus.

Then I saw there were two of them, apparently joined together by straps. The one behind was a woman, older than him. But not really old. She really did lose her footing, horribly, when she hit the ground, and rolled over. I saw her feet crazily high in the air and she crashed against a bin. I saw her head hit the pavement; heard it. The man wrenched himself free. He was holding a bag. Her bag. He held it in two hands, chest high. Somebody shouted. He ran away at full speed. He had a strange, tight smile on his face and his eyes were glassy. He was running straight for me, so I had to step out of the way. But I didn’t just step out of the way. I let the watermelon slip off my shoulder. I leaned back and swung it. I had to lean back or else it would have fallen vertically, taking me down with it. If it had continued on its circular progress around me I would quickly have lost control of it, but its progress was very suddenly halted as it hit the man full in the stomach.

They talk about the sweet spot. When I used to play rounders at primary school and I swung at the ball, mostly it would hit the edge of the bat and dribble off pathetically to the side. But every so often, the ball would hit the right place and with almost no effort, it would just fly. Cricket bats have sweet spots too, except that it’s called the “Meat.” And tennis rackets have sweet spots. So do baseball bats. And this bag-snatcher caught my watermelon right in its sweet spot, right at the perfect point of its arc. There was the most amazing thud as it struck him in the stomach. There was a whoosh of ejecting air and he just went down as drastically as if there were no body inside his clothes and they were attempting to fold themselves up on the pavement. He didn’t go down like a falling tree. He went like a tall building being demolished by explosives around the base. One minute it’s there and then there’s just dust and rubble.

I hadn’t made any plan of what to do next if the man was going to get up and come at me. My watermelon was only good for one shot. But he wasn’t able to get up. He clawed at the pavement a bit, and then we were all surrounded by a crowd. I couldn’t see him any longer, and I remembered the woman. Some people got in my way, tried to talk to me, but I pushed my way past them. I was light-headed, exhilarated. I felt like laughing or talking wildly. But there was nothing funny about the woman. She was slumped and twisted on the pavement, her face down. There was quite a lot of blood on the stone, very dark and thick. I thought she must be dead but there were odd twitches from her leg. She was smartly dressed, a business suit with quite a short gray skirt. Suddenly I thought of her having breakfast this morning and going to work, and then heading home thinking of what she was going to do this evening, making mundane and comforting plans for herself, and then this suddenly happening and her life being changed. Why hadn’t she just let go of the stupid bag? Maybe it had been caught round her arm.

People were standing around her looking uncomfortable. We all wanted somebody official-a doctor or a policeman or anybody in a uniform-to step forward and take charge and make this a regular event that was being dealt with through proper channels. But there was nobody.

“Is there a doctor?” an old woman next to me said.

Oh fuck. I’d done a two-day first-aid course in the second term of my teacher-training. I stepped forward and knelt down next to her. I could sense an air of reassurance around me. I knew about administering medicines to toddlers, but I couldn’t think of anything relevant here except for one of the key maxims: “When in doubt, do nothing.” She was unconscious. There was lots of blood around the face and mouth. Another phrase came into my mind: “the recovery position.” As gently as I could, I turned her face toward me. There were gasps and expressions of disgust from behind me.

“Has anybody called an ambulance?” I said.

“I done it on my mobile,” a voice said.

I took a deep breath and pushed my fingers into the woman’s mouth. She had red hair and very pale skin. She was younger than I’d thought at first, and probably rather beautiful. I wondered what color her eyes were, behind the closed lids. Perhaps she had green eyes: red hair and green eyes. I scooped thick blood out of her mouth. I looked at my red hand and saw a tooth or a bit of a tooth. A groan came from somewhere inside her. There was a cough. A good sign probably. Very loud and close by I heard a siren. I looked up. I was pushed aside by a man in uniform. Fine by me.

With my left hand I found a tissue in my pocket and carefully wiped the blood and other stuff off my fingers. My melon. I didn’t have my melon. I wandered back in search of it. The man was sitting up now, with two police officers, a man and a woman, looking down at him. I saw my blue plastic bag.

“Mine,” I said, picking it up. “I dropped it.”

“She did it,” a voice said. “She stopped him.”

“Fucking KO’d him,” someone else said, and close by a woman laughed.