When it came down to it, Mary was too afraid to show Kelly her tail, so she made a decision from which their friendship might never recover: she spiked her friend's drink.
When Kelly's back was turned, Mary grabbed a random vial from the box and quickly popped off the top, then poured only a few drops into Kelly's soda.
"So! What movie are we gonna watch?" Mary asked as Kelly came to sit beside her on the sofa, picking up her soda. She took a long drink, downing easily half of the liquid.
"Oh, just a com--what the heck is *that*??" Kelly exclaimed, eyes fixed on the end of Mary's twitching tail that had escaped from her t-shirt where she'd been hiding it wrapped around her torso. Mary searched for words but couldn't find any. Instead, she just stood up and unwrapped her tail, holding it in one hand as it dropped to the floor.
"I'm--wow," Kelly said, finishing the rest of her soda in one gulp. "I'm gonna need something stronger to deal with . . . whatever this is." She gestured to Mary and took her glass back into the kitchen.
It was mere moments after that Mary winced as she heard a glass shatter on the floor and Kelly shriek. Meekly, Mary padded into the kitchen and saw her friend standing there with two slowly unfurling butterfly antennae on her forehead. They were dark and getting longer by the second and began to twitch as they grew, flooding Kelly with new sensations.
"Don't be mad--" Mary began, her tail thrashing.
"You did this?" Kelly said, tentatively fingering one antenna. Mary nodded.