Lisa could feel a strange sensation all over her body. Returning the book to her bag, she stretched out on her beach towel wait for the changes to begin. The sun was felt pleasantly warm, especially on Lisa's legs. She closed her eyes and let her mind drifted from the spellbook to focused on the warm, relaxing sensation.
Lisa wore a modest blue one-piece swimsuit. As she lay there, the bottom of her suit started to stretch, slowly flowing down her legs like oil. Her legs drew together, the magic wrapping together as a single appendage. This continued on down her ankles and heels and down to her toes. If anyone had paid attention at that moment it would look as if Lisa had on the bottom half of some kind of strange wetsuit.
Then the next wave of changes began, moving in the opposite direction. Lisa's feet flattened out into a tailfin and took on a lighter shade of blue. The change worked its way up her legs - which were now a tail. As it went, the swimsuit-like material seemed to melt into Lisa's skin, leaving behind a pattern of shiny blue scales. The scales stopped abruptly at her waist, but changes continued up Lisa's body. Her one-piece melted away leaving her with an athletic and lightly-tanned stomach. At her breasts, the remaining portion of the one-piece became a bikini top which stretched slight as Lisa's breasts expanded from the forgettable A's to firm C's.
"Lisa..."
She opened her eyes at the sound of Matt's voice, having apparently returned from his beach stroll.
"Yes?"
"You're...you're..."
Lisa began to sit up, her legs feeling unfamiliar. Then she saw it: her tail. Matt's dreamgirl was...
"...a mermaid" Matt finally stammered.