Deal with the devil

"Well, my dears, I have a potion here that just might help you, but..." the old woman trailed off, grinning darkly at them. "But what?" they asked in unison. "But that's all I'm going to tell you," she said. "If you want it, take it and drink it. It's quite free. And if not, then get out of my store." The Joannas swallowed hard and looked at each other. "Now what?" said the right one. "I don't trust her," said the left. "I don't either," replied the right. "So what do we do?" asked her twin.