"You are really bad at guessing," Derek teases. Bitty's face does a complicated series of expressions that Derek can't even begin to decipher when he mentions the party, and that doesn't really give Derek much hope.

"Okay, well," he starts, sliding his hand into his pocket. "Magnus gave me a gift for Christmas. It's the same potion, but it makes anyone who drinks it a wolf for a few hours."

He pulls the small vial out and holds it out in his palm, offering it to Bitty. "I was wondering if you wanted to try it."