I replied, "Maybe that's all that they had?"
She said, "You don't do that with root beer. Oh, that's all that's left, I'll have a root beer. You have to like it."
I considered that. "Just like you think that Dr. Pepper has prune juice in it?"
Then I thought about prune juice. A prune is a dried plum. If it's been dried, then where do they get any juice from? It's not like you can drink 'raisin juice'.
Of course, wikipedia knows all.
Prune juice is made by softening prunes through steaming and then putting them through a pulper to create a watery puree.
Interestingly enough, that page also points out that the Dr. Pepper assertion is a long-standing urban myth.
So, prune juice is actually a dried plum puree. Enjoy!