The instructor put his hand on the butt of the _Thor_ gun. The native walked past the group without seeming to see it. He was whistling between his teeth. He walked on ahead of them, turned down an alley, and disappeared. The instructor took his hand off the _Thor_ gun. "He wasn't really following us; he wouldn't dare. Does anybody have any questions?" He looked brightly around the group. "Yes, I have," Joe Buckner said. "Why don't we just kill all of these natives? They're not any good to us." The instructor smiled slyly. "I'll tell you a little secret about that. It's awfully hard to kill _all_ of any race. No matter how thoroughly you do the job, a few always manage to escape. Then they breed and increase in spite of everything you do. "After we had conquered this planet we had trouble catching all of the natives. They were the most cantankerous, persistent race you can imagine. So these museums were set up, to lure them in here. We announced that these places would be set aside and that they would not be bothered as long as they remained in the museums. All in all, we made the museums rather attractive places, hoping that--" "I see the plan!" Joe Buckner said glowingly. "After you got them all into the museums--_blooie!_--knock all of them off at once!" The instructor smiled. He looked as pleased as if he had thought of the idea himself. A little stir of applause ran through the group as they expressed their gratitude to their rulers for making this world safe for them. "Why haven't they been killed before now?" Billy Kasker asked. "These museums were opened over forty years ago. Surely--" "I don't know about that," the instructor answered. "I think probably our rulers are waiting for a propitious time, or perhaps for an incident that will give them an excuse to carry out their plan." "I hope they don't wait too long," Joe Buckner said. "Golly, I want to be a _Thorgunner_ and get in on the mop-up when it comes!" The group stirred, seemed to look forward to the day of the final slaughter. "Any other questions?" the instructor asked. "I have one," Billy Kasker said hesitantly. "It doesn't exactly have anything to do with our trip through the museum--it's something I ran across in a book--but I don't quite understand it, and I wondered--" "Go right ahead, Billy. What do you have on your mind?" "Well, ah, did--did you ever hear of a _changeling_? I know it's a kind of a silly question but--" "A _changeling_?" The instructor frowned. "I think it comes out of a fairy story or something like that," Billy Kasker said. "Oh, yes. Now I recall the word." The instructor's face lighted. "It's a story about the fairies taking one child from its crib and substituting another for it. The substituted child was called a _changeling_. Or perhaps some poor mother, wishing to give her child a better chance, stole the child of a rich mother and put her child in its place. I really don't remember too much about it."