The girl crouches at the edge of the rock and looks in.
It is smaller than a bathtub. The water is cold and perfectly clear. A strand of kelp, dark as wet leather, sways without any current she can see. Two limpets sit on the far wall doing nothing she can detect. A hermit crab pauses at the bottom, one claw raised, as if it had been about to say something and thought better of it.
Her mother calls from the car park. The girl does not look up.
Under the kelp there is a patch of something purple. She doesn't know the word coralline and won't learn it for another six years, in a marine biology elective she'll take because she remembers this afternoon and can't explain why. The purple thing is alive. It is growing so slowly she will never see it move.
A wave hits the rocks below and a fine spray reaches the pool. The water shivers. The hermit crab lowers its claw and picks its way across the sand. The limpets do not react. They are doing something she cannot see — filtering, breathing, holding on.
The pool is a world that doesn't know it's temporary. Twice a day the ocean comes back and unmakes it — pours in, swirls everything, takes some things and leaves others. Then the water recedes and the pool is there again, the same shape, mostly the same inhabitants. The limpets haven't moved. The kelp still sways. The purple thing is six hours older and no larger.
It is not the same water. It is the same pool.
Her mother calls again.
The girl reaches in and touches the surface. Her finger makes a tiny dip, barely a ripple, and the cold bites. The hermit crab freezes. The kelp sways the same. She pulls her hand back and wipes it on her shorts and the pool is exactly as it was, except now it has been touched, which is different from not having been touched in a way that leaves no mark.
She stands up. Her knees ache. She's been crouching longer than she thought.
On the drive home she falls asleep in the back seat and dreams about something else entirely, and the tide pool stays where it is, filling slowly from below, and the hermit crab finds something worth eating, and the limpets breathe, and the purple thing grows.