Athena was in Sarah's body and thought she was Sarah.
Which meant she remembered doing everything Sarah did.
Which means she remembered making the accidental wish and panicking.
"Wait a minute," said 'Sarah'. "I just made a wish by mistake. I should have swapped with you! Why am I still here?"
"No, you twit," said the real Sarah in Athena's body, "I just made a wish by mistake. I did swap with you, and I'm in here!"
"You're telling me you're really me?" asked 'Sarah'. "I'd sooner believe you cast one of your spells to keep safe of wishes." Goths didn't cast actual spells, of course, but Sarah ignored that... so so did the new 'Sarah', who thought just like the original.
"There's only one Sarah, and it's me," said Sarah.
"Yeah?" replied 'Sarah'. "I've got the body, and the money, and the guys. You don't look like me, and nobody would say you are me. So you can't be me!"
"That's all mine!" yelled the other Sarah. "The body, the money, the guys, and one other thing... the stone."
"That, I know you don't have," said 'Sarah', holding up the stone. I wish you'd tell me why you're pretending to be me."
The stone flashed and both girls had to look away for a second. Then Sarah (in Athena's body) said "I'm not pretending to be you, I am you. You're Athena."
"What, that didn't work?" said 'Sarah'. "I don't see how you could be telling the truth... but the magic made you tell the truth. Argh. I wish I just knew how to solve this whole thing.
