About 6 minutes to read aloud.

The box was wedged behind an old water tank, under a tarpaulin gone stiff with dust, and Nell had to lie flat on her stomach to reach it. It was Christmas Eve, and she had gone up to the attic looking for the tin of glass baubles her mother kept from one year to the next. But her fingers found wood first, smooth and cold, and she pulled it out into the light from the little round window.

It was a box, no bigger than a shoebox, made of dark wood with a brass clasp gone green at the edges. There was no name on it, no date, nothing carved into the lid except a single star, worn almost flat by handling. Nell carried it down two flights of stairs and set it on the kitchen table, where her grandmother was rolling out pastry.

"Where did you find that?" her grandmother said, and stopped rolling.

"Behind the tank. Whose is it?"

Her grandmother wiped her hands on her apron and turned the box over once, slowly, the way you'd turn over a stone you half remembered from somewhere. "It was in the attic when I was your age," she said. "My mother said it was there when she was a girl too. Nobody in this family has ever known whose it was to begin with."

"Has anyone opened it?"

"Not that I ever heard. The clasp always looked stuck fast."

Nell tried it anyway, and it gave. Inside, packed in neat rows on a bed of faded velvet, were dozens of small paper stars, each one folded from a different scrap of paper: a torn corner of a letter, a strip of old wrapping, a page from a ledger with figures still visible in brown ink. Every star had a name written across it in a different hand, and a year beneath the name.

The oldest one Nell could find read Thomas, 1888. Others followed close behind, decade after decade, the ink fading and the paper yellowing as the years climbed forward: a Margaret, a Walter, an Iris, a Frank. Near the top, in blue biro, was one that read your grandmother's grandmother, 1962, and Nell's own grandmother went very quiet when she saw it, and touched it with one floury finger.

"I didn't know it went back that far," she said.

"What are they for?"

Her grandmother thought about this for longer than the question seemed to need. "I think," she said eventually, "that whoever found the box first must have started it, and then it got left somewhere for the next person to find, and they added their own without quite knowing why either. It isn't the sort of thing anyone writes down the rules for. It just keeps happening."

Nell counted the stars twice and made it forty-one both times. She thought about all the people whose names she didn't recognise, sitting in rooms in this same house on Christmas Eves she hadn't been alive for, folding a scrap of paper because someone before them had done exactly that. She thought it was a strange thing to feel fond of people she had never met, but she did.

There was one blank space left at the bottom of the velvet, just large enough for one more star. Nell asked her grandmother for a pencil and a scrap of paper, and after a while she settled on the back of an old shopping list, because it was the first thing to hand and because, she decided, that seemed to be rather the point of the whole thing: whatever paper happened to be nearby when you needed it.

She wrote her name and the year, folded the paper into a star as best she could manage, and set it into the last space. It didn't sit quite as neatly as the others, one point a little longer than it should have been, but it fitted.

"What happens to it now?" she asked.

"I expect," said her grandmother, "someone puts it back behind the water tank, and it waits for whoever finds it next."

Nell thought about arguing that there wasn't room for a hundred more stars, and that whoever came after her would have nowhere to put their own name. But she didn't say it. Some problems belonged to the person who found them. She closed the lid, did up the stiff little clasp, and carried the box back up the two flights of stairs herself, to wedge it behind the tank exactly where she'd found it, in the dark, under the tarpaulin, where it would wait.