Lydia lay on the carpet next to you, likewise soaked from the rain. She was breathing a little heavy, which seemed unusual: yes you'd both just sprinted through the rain, but she was rather athletic.
"Lydia, are you okay?"
"I'm great" she said. "I just wasn't expecting it to rain. The forecast this morning was clear."
"Well, weather is like that."
"Yeah, but I was hoping to have a better chance to surprise you."
"Surprise me with what?"
"My new swimsuit."
That's when you saw a sea-green spandex working its way down Lydia's legs past the bottom of her skirt. You both watched her legs draw together as the material steadily worked its way down her legs. She scrambled to get her shoes and socks off before the transformation reached her feet.
As Lydia's feet changed into a tailfin, you heard a slight rip of fabric and realized your own legs were still straining at your pants - the only barrier between you and a tail of your own. Unable to take it any longer, you unzip your pants and pull them off. The silver spandex has already covered your legs down to your ankles. The moment there is no longer a clothing barrier between your legs they stick together and merge into a tail. You hurriedly remove your socks and shoes as the material spreads down your feet which then form into a tailfin.
You feel the pressure on your body ease as you look over your new tailfin, your transformation into a merman apparently complete. Your silver tail reminds you of a dolphin. You look at Lydia's new tail which is reminscient of Ariel's, minus the scales. Your eyes meet Lydia's who has clearly been eagerly watching your transformation.
"So does the mermaid like her new merman?" you ask.
"She does."