"How do you know I'm a mermaid?!" asked a shocked Lilia.
"The other mermaid told me" explained Kevin. "I had no idea until then. I wouldn't have believed her if she hadn't been a mermaid herself."
"The necklace lets you have legs?"
"Yes. The other mermaid, Gisela, had been using it and left it behind."
"Show me."
Kevin sat down on Gisela's couch and took off the necklace laying it on the coffee table.
"I don't know long it will...oh!"
The sensation of the transformation was surprising, especially out of water. Kevin felt his legs pull together, the transformation working its way down from her crotch. As the legs pulled together, the golden scales soon followed traveling the same direction. Kevin put her legs up on the table as her feet flared out into fins.
"You really are a mermaid now." Lilia had remained a little skeptical about Kevin's story until now. A little stunned, she walked over to the couch and sat down next to her boyfriend-turned-mermaid.
Something occurred to Kevin: "You don't wear a necklace. How do you get legs instead of a tail?"
"I take a serum. So how did this mermaid change you?"
"Gisela kissed me while in mermaid form and somehow that caused us to swap who was the mermaid and who was the man."
"You kissed her???" said Lilia, clearly upset.
"It's a long story..."