At the bottom of Nettle Garden, between the lavender and the old stone wall, lived a snail called Moss.

Moss was not a remarkable snail in most ways. He was small. He moved slowly. He ate the occasional leaf and left a trail of silver wherever he went, which the spiders found annoying but the moonlight found rather beautiful.

But Moss had one thing no other snail had ever had: a worry shell.

It had come to him the way most important things come — without warning, one ordinary Wednesday. He had woken up to find his shell had turned a deep glowing green, and that when a child leaned down and whispered a worry into the opening, the worry would travel inside, curl up small, and stay there — leaving the child light enough to sleep.

Every night, just after dark, the children of the neighbourhood came one by one. They crouched in the grass, pressed their lips close to Moss’s shell, and whispered.

“I’m worried about my spelling test.”

“I’m scared of the noise the pipes make.”

“I think my best friend might be cross with me.”

Each worry floated in, settled, and the child went to bed free of it. And every morning, Moss breathed them all out into the dew, and they dissolved — small, harmless, already shrinking in the daylight.

It was, all things considered, a very good system.

Until one October night, when a small girl named Wren arrived last, crouched down, and whispered a worry so big it barely fit through the shell’s opening at all.

Moss felt the shell shift on his back. It was heavier than it had ever been. The glow it usually gave off — the calm, steady teal-green — was flickering at the edges, the way a candle flickers when the room has too much wind in it.

Wren noticed. She sat cross-legged in the grass and peered at him with solemn eyes. “Are you alright?” she asked.

Moss considered. He moved one eye-stalk to look at his shell, then moved it back. “I believe,” he said — for Moss could speak, though very slowly, the way important truths tend to — “that I am nearly full.”

“Can shells get too full of worries?” Wren asked.

“I have never found out,” Moss said. “Until tonight.”

Wren thought about this for a long time. She was the kind of child who took things seriously without being heavy about them — the same way some children carry laughter lightly, Wren carried thinking lightly.

“What happens to the worries in the morning?” she asked. “When you breathe them out?”

“They dissolve into the dew,” Moss said. “They become water. The garden drinks them. The lavender grows a little taller. Nothing is ever wasted.”

Wren looked at the lavender. It was, she thought, unusually tall for October. A whole summer’s worth of children’s worries, turned into something purple and sweetsmelling. She found this extremely comforting.

“Then what if,” she said slowly, “instead of giving you another worry tonight — I took one back?”

Moss went very still. In all his years of collecting worries, not one child had ever offered to give something back to the one who carried things for others.

“Which worry?” he asked.

Wren pressed her lips together, thinking. Then she reached into the shell’s opening — carefully, the way you reach into something sacred — and she drew out not a worry at all, but a small warm thing, round as a coin, that glowed.

The thing in Wren’s hand was warm and small and smelled of rain on warm stone. It did not have a name, exactly. But it felt like the moment just before sleep when everything goes soft and the day stops mattering and the body remembers it is safe.

Wren looked at Moss. “Can I keep it?” she asked.

“That,” said Moss, “is what it is for.”

Wren closed her hand around it and walked back inside. She did not give Moss her worry that night. She found, peculiarly, that she didn’t need to — holding the warm thing was enough. The worry was still there, somewhere, but it had become small and unimportant, the way a word stops sounding like a word if you say it enough times.

She got into bed. She opened her hand one last time. The small golden thing glowed in the dark of her room — gently, steadily — and then went out, like a candle that had done its job.

Outside in Nettle Garden, Moss’s shell had lightened. He moved a little more easily across the stone. He looked up at the sky — at the stars, which were each, in their own way, someone’s worry turned into light — and then he tucked his eye-stalks in, and slept.