She started as the rain fell, twitched and gasped as the first drops hit her sand-coated skin. Caught herself and breathed out slowly. Found her fingertips and ran them up her sides, along the slight sliminess of her swimming costume. Dabberlocks and sugar kelp, her first conscious thought, a nice touch.
Sitting up, she brushed the dried sand from her arms to reveal skin the colour of damp sand, dimpled like measle scars from the rain that had landed before she woke. Ran her fingers through her thongweed hair and turned her attention to her tail.
The child who'd made her had taken great care, collecting the prettiest shells to give that tail beautiful colour, but - and for this the mermaid felt most grateful - the child had also realised that a tail was impractical on land, which meant the mermaid could wriggle it off.
The lap of every wave upon the shore was a tug against her soul, but tonight would be her only night on shore so she resisted the sea's call. All the same, as soon as her toes were free she plunged them deep into the sand, seeking the damp. She grinned, wrinkling her mother-of-pearl eyes, and threw herself to her feet, stretched one leg out and pointed her toes to dance and pirouette in the rain, past castles and mountains made from the sand. Reaching down, she grabbed handfuls of sand to toss in the air before running through the surf, jumping over and kicking the waves.
When she paused to catch her breath, standing knee-deep in the sea's susurration, she saw the other beach creations nearly all gone, collected by their new homes in the wind or sea, and she felt a pang of melancholy as she realised her own time was short. Her walk back to her tail was slower than her dash away from it, and she breathed out slowly as she scooped it up.
It was beautiful, iridescent with pinks and greys and greens and purples from the shells and seaweeds used, but she noticed the limpet and barnacle shells had, rather than being absorbed into her tail, instead returned to life and attached themselves to it. She smashed them off, irritation fading as she swallowed their flesh. Coming to life had made her hungry. By the time she'd cleared them all, the clouds and cleared. She held her tail up in the moonlight to make sure she'd missed none, and the sheen and colours captivated her again.
She walked back to the sea and sat down when it reached her waist to wriggle her tail back over her legs, and with no backward look she swam out to sea, out to the castles and where her kind waiting for her, leaving nothing but the smashed shells and her footprints.