Ava is studying the ocean. She looks down at her brother, growing all the time, losing the baby roundness she has secretly cherished, but still small and vulnerable to her ten-year-old eyes. His armbands are at the bottom of their mother’s beach-bag. He cannot yet swim without them.

“Shall we go back to Mum and Dad?” she asks heroically. “We can build a sandcastle if you like.”

“Let’s explore the rock first.”

“How can you explore a rock?” she asks, then adds, “We’re not climbing it, it’s all right for me but you’re too small,” but he’s already off, disappearing before her eyes, that steady sturdy tireless jogtrot she finds so hard to keep pace with. A second later and he’s back.

“Come and look!” he’s yelling. “It’s this really ace weird thing!”

She’s reluctant to get too excited, knowing his definition of an ace weird thing is not the same as hers.

“What is it?” she asks, not moving.

“I don’t know, but it’s ace! Come and see!”

Just in case she’s being watched by anyone important, she rolls her eyes and sighs before following him. On the rock’s seaward side, there’s a large rectangular hollow, like a giant rock-pool. It’s big enough to swim ten or fifteen strokes in. It’s swimming-pool-shaped, but the bottom is sandy and greenish weed floats back and forth like drowned hair.

“What is it?” he asks.

“I don’t know!” she snaps, annoyed because she likes knowing the answers and being able to tell him. “Why do you always expect me to know everything?”

He shrugs. “Mostly you do.”

There are people in the pool, although they don’t look as if they’re enjoying it. A child floats precariously in a thin inflatable ring. The frill of her swimming costume flutters as she kicks. The child’s father sits on the side and berates her for being scared.

“Let’s go in,” Finn says.

“You haven’t got your armbands.”

“I’ll just go in a little bit,” he replies, clambering up.

“You can’t swim yet.” She can’t imagine why he wants to go in anyway. To her it seems like the worst of both worlds, the clinging communal oiliness of a public swimming pool combined with the chill and salt of the ocean. Why not just walk the extra ten feet and get into the proper water? “You’re not allowed, Mum and Dad said so.”

“I can swim without my armbands,” he says. “Look, I can do it.”

And to her astonishment, he almost can. His head bobs beneath the water and his foot slips secretly beneath him to push off from the bottom every few strokes, but he’s undeniably making progress, huffing and breathless and jerky, but not sinking, not drowning. He’s become a fledgling water-creature.

“You can!” she says. “You can swim. You’re swimming! When did you learn that?”

The pride in his face as he glances shyly towards her squeezes her heart.

It takes them a long time to find their way back. She keeps up a bright artificial stream of chatter in case he’s worried, this little boy she loves and tyrannises so fiercely. After a few minutes, she feels a warm hand slip trustingly into hers, and she squeezes it, breathless with responsibility.