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"Hey," I said, returning to my chair, "You just told me a lot, so now I should tell you a few things. Fair?" She shrugged. I reached into my employer's desk drawer and pulled out a pen and paper.
"I'm going to make my guess about what's so interesting about what Fifteen said, and I'm going to write it down."
"Like you do," she said. "So you can prove to Mom that you got it right."
"Exactly. Fifteen told us Tracy's age, and how she was dressed, but he didn't know who she was or where she was from."
"Of course not. He's not a detective."
"Ah, but here's the thing. People in U-town mostly don't carry identification. We don't need it. But in the rest of the world, people usually do carry ID of some sort, especially when they're traveling. Fifteen is very thorough, and if Tracy had had any ID on her, he would have found it. Plus, she had a suitcase, and if you travel by plane or train or bus there's usually a sticker or a tag that they attach to your bag. If that had been there, he would have known where she'd come from, even if he didn't know her name."
"So, how do you think she got here?"
"Oh, one of those ways, probably, though we do need to find out if she was traveling alone. But I'm not just guessing. My memory isn't as good as your mother's, but I did see that suitcase for a moment, in the room, and there was a tag tied to the handle. So, that means that somebody removed the tag, and probably took whatever ID she had on her. The logical assumption is that it was the murderer. And the next logical assumption is that you didn't kill her. We already knew who she was and where she was from, and you knew that we knew. So, you'd have had no reason to take anything."