Branwen's journey for a quiet study space had led her further afield than usual. While she was aware that the Slytherin Common Room was around there somewhere, she rather doubted there would be too many people in the other rooms.
It was just her luck that the first door she pushed open revealed someone swearing - but not at her, at a bright blue potion that she didn't think she had seen before. As she tried to identify the potion, she hardly noticed the girl making it, but she had only the vaguest of recollections of seeing her once or twice in the hallways before.
She waited until the potion had calmed, before saying curiously, "I'm sorry. I don't mean to disturb you, but I have to ask. What are you making?" To back up her claim, she continued to stand at the door, intruding as little as necessary.