Serpentine Gallery 24-hour Experiment Marathon

They'd paid a sailor two hundred dollars to stow them away. He was fat, and had a stripey shirt, like James had always pictured sailors wearing. The sailor took the money from Gloria and told them to come to the boat at midnight: The Insurgent Dawn, Quay Twenty. He promised them a secret cabin, but when they arrived and picked him out among the wagons and containers sitting idly beneath arclights, he led them across the deck then down: past cabins and store-cupboards, engine rooms and medical depositories, right down into the vessel's bowels - then, when it seemed they could descend no more, jerked back a rusty trapdoor.

'What's through there?' asked Gloria.

'Oil tank,' the sailor answered. 'Climb down the rungs just under here. You'll find a boat tied to the bottom one. That's where you'll stay. I'll come and find you when we dock.'

They did so. It was a small boat, like the ones the fishermen used to sit in on the lake near the village. On its floor they found a blanket and a plastic ring.

'What's the ring for?' Gloria shouted up at the sailor.

He closed the trapdoor above them, shutting out the light. James started crying. His sobbing echoed off metal walls: it echoed, and the echo echoed too, running over the surface of the oil from one side to another. James fancied he could feel each sound-wave passing him, rustling his hair like wind; after a while he put his theory to the test by letting out a sob louder than the others, then sitting up and waiting for it to bounce back past him. 'It sounds like there are twenty of us in here,' Gloria's voice said in the darkness by his ear. 'We can pretend there are.'

Her words, too, echoed back.

'Hello!' she called out. 'What's your name?'

She waited a little, then called out in an altered voice: 'Bella!'

'Hello Bella!' Gloria answered herself in a third voice when the echo had died down. 'I'm Simona!'

James joined in. For the next few hours they peopled their dark world with children, orchestrating dialogues and duets, arguments and alliances, birthday parties, football matches, school assemblies. Then they were tired, and slept beneath the blanket. When James woke up, it was still dark.

'I need to pee,' he told his sister.

She was asleep. He prodded her awake.

'Do it over the side,' she said.

He did. It didn't sound like peeing into water: there was no trickle, no plash - just a hard, leaden sound. 'I can't get back to sleep,' he told her afterwards.

'Well, lie there and think of something,' she said.

'Think of what?' he asked.

'I don't know,' she said. 'Think of home.'

He tried to picture the kitchen: the wooden shelves above the cooker, and the pots on them. But he couldn't make them form amidst the nothingness around him.

'Gloria?' he asked. He could tell from the rhythm of her breath beside him that she was still awake.

'Yes?'

'Why did we have to leave?'

'Because of the war,' she answered.

He lay in silence for a while, then asked her:

'Gloria?'

'What?'

'Where we're going, is there no war?'

'The war's everywhere,' she told him.

'If the war's everywhere, what difference does it make to go from one place to another?' he asked.

'Where we're going,' she said, 'we can sell things. We can sell things to the soldiers in the city, and the other people too. And with the money we make, we can buy food.'

'Will we make lots of money?'

'If we work hard, yes.'

'Can we buy cakes with it?' he asked.

'If we make lots of money, we can buy lots of cakes,' she said. 'But go to sleep now.'

Lying on his back, James silently gathered all the echo-children they'd created, summoned them into the boat and hired them to work for him. They'd go around and sell things - watches, say, or bags and telephones - and bring the money back to him and Gloria. Gloria would be his secretary, and he'd be the boss. He'd pay the other children with the money they brought in, then send them out to buy him cakes. After a few days' trading, he'd have so much money that he'd be able to have a new desk built for him entirely out of cake-material: a desk, and a telephone as well, both made from cake. If he felt peckish then he'd take a chunk from one of its legs, or eat an empty draw; to end phone calls he'd just bite the mouthpiece off.

When he woke up, hunger was pushing at him from inside, making his stomach's walls expand. They ate some biscuits Gloria had brought, then slept again, then woke up hungrier than before. The hunger grew in him until it seemed that it was bigger than his stomach was, seemed that the rest of him was hovering above a tank of echoing black emptiness. Eventually he reached his hand over the boat's side and dipped his finger in the oil, then drew it to his lips, willing himself to taste its texture on his tongue as cake, or even biscuit. Soon after this the darkness around him started to jump and swirl, its particles separating and then recombining in a dance that might have been quite pretty if there had been any colours. The voices of his and Gloria's children gave over to deeper, adult ones, speaking in conspiratorial whispers, discussing their fate. He spent the last stretch of the journey neither conscious nor unconscious, neither alive nor dead.

When the sailor opened the trap-door again their hold was bathed in the light of a million suns. It was so bright they had to turn away from it and cover their eyes as though cowering in awe before an angel. He shouted down at them to wait a little longer, until dark, when he would come and fetch them out, set them ashore. He kept his word. The new quayside was covered in snow. As they made their way unsteadily across it, they left black, oily footprints. They walked inland, and found discarded food in a skip beside a supermarket, which they threw themselves on, tearing off the packaging like lions ripping zebras' stomachs open. After they'd eaten all they could, they wiped their oily hands against the skip's side, then looked back at the trail they've made.

'It wouldn't be too hard to track us,' Gloria said. 'We're like snails.'

'Or slugs,' said James.

'Same thing,' said Gloria. 'A slug is just a snail that has to keep on moving because it's got no home.'

'But snails have to keep moving too,' said James, 'and carry their houses with them, on their backs. That's worse.'