This is theis is t Rock. Most of the time, it is a small, craggy island, with grass clinging to it where possible. There are level places here and there, and gardens. Buildings have been put up where possible: houses, hostels, hospitals, schools. The sizes vary considerably, suggesting that not all the people who live here are the same species.

This is the space between stories, the metafictional refuge which all characters escape to at some point. Perhaps they come here to escape when life in their own narrative becomes unbearable. Perhaps they come just to take some time to process the memories they have lived through, or to make friends with people who have had similar experiences in other stories. Whether or not they came here in life, they retire here when they die – or when they finally pass on after a period of being undead, if the presence of a ghost or a zombie is necessary to the plot.

But at low tide, the Rock is no longer an island. The water retreats and reveals the causeway to the mainland. And here there are no permanent residents except the trees, the grass, and the guinea-pigs, who have been breeding here ever since a wizard's experiment sent a pregnant female into an uninhabited space with no way of getting home, and, more importantly for her, no danger of being subjected to any more experiments. In between the trees, there are pools, and each one leads to a different world.

Back on the Rock, a house with a two-part door, like a stable door but taller, opened its top section. The centaur glanced down at the small boy who stood outside. The child was a slender ten-year-old with a mop of shaggy blond hair, and golden eyes flecked with brown.

'Morning, Coin,' the centaur said.

'Good morning, Master Cheiron.'

'Want to come in?'

'No. I want to go out.'

'Okay, but let's talk about it first, shall we? Cheiron opened the lower half of his door. 'Want a biscuit? Anything to drink?'

'Just milk, please.' Before Cheiron could pour anything out for him, the boy conjured himself a glass of creamy milk. It was unpasteurised, but so pure that it didn't need anything to kill pathogens. It was as warm as if it had come directly from the cow, if any cows had been involved.

In his other hand, the boy produced a mug of tea, which he handed to Cheiron, who thanked him and sipped it. It tasted perfect. Anything Coin produced was perfect.

Cheiron knelt on a mound of straw to be closer to the boy in height, and gestured for Coin to sit.

Coin remained standing. 'I think it's time to move on,' he said.

'I thought you were going to retire to the world you'd created,' said Cheiron. 'That's what sourcerers usually do.'

'It was lonely there,' said Coin. 'It's funny – I didn't know I was lonely, before. I didn't have anyone, except the voice in the staff talking to me. I didn't know any other children. So I didn't know about having friends. But now…' he waved his free hand in frustration. 'I didn't know how to live on the Disc, if I wasn't going to rule the world any more. And I don't know how to live in my own world.'

'Could you have gone back to the Discworld? Made everyone forget you were the Sourcerer, made yourself stop being a sourcerer, and just enrolled as a normal student wizard?'

'No. Because I didn't know how to be a normal boy. Nothing in my life had ever been normal! Surviving when both my parents died when I was a baby isn't normal!'

'Well…' Cheiron began.

'Oh, if I'd been adopted by an aunt or something, and found out I was magical when I was older, and started school, and then found out that I was more than just an ordinary student wizard, that would've been all right. Or if I'd been adopted by dwarves and grew up not knowing I was a human and not understanding why I was taller than all the grown-ups, that would still have been – fairly normal. But being brought up by an octiron staff with my father's voice in it always telling me what to do? I don't know anything about how to be human!'

'So, what do you want to do?'

'I reckon,' said Coin thoughtfully, and his voice was starting to sound more rural, more like that of an ordinary village lad than of a boy whose only company has been a wizard's ghost, 'Seems to me, you need to learn to be ordinary first, before you can be extraordinary. I'd need to go back to bein' a baby, an' get adopted by normal parents. I might have friends! An' a dog. I always wanted a dog. There was this dog I saw, when I came to Ankh-Morpork. An ole stray, begging for scraps, an' bits of his fur were falling off. I wanted to make him into a talking dog, so I could ask him if he'd be my friend. But the staff said there was no time for that, and when we made the world perfect it wouldn't be cluttered up with mangy strays.'

'Would you like to have a talking dog?' asked Cheiron. In his time visiting the Rock, Coin had met plenty of strange creatures apart from centaurs, including a genetically engineered talking wolf.

'No. Not now. Jus' an ordinary dog. But a little scruffy dog like that dog, with a funny ear that looks inside out. An' I don't want to be magical until I'm old enough to decide for myself what to do with it. Well – maybe a bit magical, but not too much. Not too powerful, until I'm old enough.'

'How old is that, do you think?'

Coin was ten. As long as he remained on the Rock, he could not grow older than ten. 'Eleven,' he said firmly.

'Fair enough,' said Cheiron. 'It won't be easy,' he warned. 'You've still got that potential in you, the risk of causing the end of the world.'

'Like the ice giants? I didn't know they'd come, I jus' thought the gods weren't doin' a good job of runnin' things an' I could do better. The staff never told me about the ice giants!'

Coin was on the verge of tears, and Cheiron gave him a reassuring hug. 'Maybe your father didn't know about the ice giants,' he said, when the boy had calmed down. 'Or maybe he thought you'd be more powerful than them, or than the Things from the Dungeon Dimensions.'

'Yeah, or maybe he jus' didn't care as long as I showed up those ole wizards who didn't want him to get married,' said Coin bitterly.

'Yes, or that,' Cheiron agreed.

'I hope I get adopted by someone sane, next time. An' kind. But mostly, sane.'

'Agreed.'

'An' I'm not startin' any more wars, next time.'

'No, but what if there was a war going on anyway? Between – well, another world's equivalent of the gods and the ice giants?'

'I'll tell 'em to stop – no,' admitted Coin. ''Cos if I did that, it'd mean I was rulin', an' that didn't work out well before. But – I'll think of somethin'.' He thought. 'There were – four horsemen, weren't there?' he said. 'I didn't see 'em, because they forgot to turn up, but they were meant to. Only they went into a pub for a drink, an' forgot where they were supposed to be goin', an' then their horses got stolen.'

'That's right.'

'Are they comin' back? I can see 'em again, only – they look different, an' one of them's turned into a woman, an' they're riding – sort of metal horses, with wheels instead of legs. Two wheels, one in front of the other. How do they stay up?'

'I expect you'll find that out. But yes, whatever world you go into, they'll probably follow you there. They'll be waiting to see whether you actually do end the world this time.'

'I'm scared,' Coin admitted.

'I'm not surprised. But when you get there, you won't remember that there are all these things to be scared of. You'll be a baby, and – well, being a baby is scary enough in itself, but you'll soon find out that it's all right and you have parents who love you and take care of you. Would you prefer to be the only child of a rich important international diplomat or second child of an accountant?'

'The accountant's son.' Coin wasn't sure what an accountant was, but it sounded safely boring.

'Good choice.'

Cheiron set off, with Coin following behind him. Centaur and boy walked out to the woods, until they found a suitable pond. It looked only a couple of feet deep. Coin dived in, and disappeared.

Far away from the Disc is a world which looks very different, which is said to have been created by an experiment at Unseen University. Its inhabitants call it Earth. On a considerably larger island than the Rock, in a hospital run by Satanist nuns, a blond-haired baby materialised alongside two other very similar-looking babies. As far as anyone except the nuns knew, he was an ordinary human, a descendant of the first creatures to transform from inanimate earth into living creatures. So he was given a name which means both human and earth.

His story began.