"Thank you, sir. You have explained it very lucidly." The instructor beamed. Joe Buckner sniffed. "Asking a question, then telling the instructor he has explained it very clearly when you didn't even ask a sensible question in the first place--that's what I call sucking in! Who ever heard of a _changeling_?" The group moved on. They came to the section of the city that had been repaired. The streets had been cleared of the rubble, houses had been rebuilt, and here and there little touches of green grass showed where an attempt to add a touch of beauty had been made. They saw very few of the natives. Far ahead of them they occasionally glimpsed a native slipping furtively out of the way. Behind them, always at a distance, heads occasionally poked around corners at them. "They're very cowardly," the instructor said. "Where's Billy Kasker?" Susan Sidwell suddenly asked. The group halted. Billy Kasker was no longer following them. A little stir of consternation ran through them as they realized the class president was missing. "Billy! Billy!" the instructor called. There was no answer. "I just don't understand this. He knows he should remain with us." "Maybe some of these horrible natives grabbed him!" Susan Sidwell said. The group was startled--and suddenly afraid. * * * * * The instructor took a deep breath. "I have a _Thor_ gun. I'll go find him. Joe, you are in charge of the group until I return. All of you remain in the middle of the street and don't move." The instructor went back along the street. He was exasperated and a little alarmed. If anything happened to Billy, how could he explain the matter to the gate captain or to Billy's parents? "Billy!" he called again and again. Suddenly he had an answer from an alley. "Here, sir--here I am. Are you looking for me? I'm sorry, sir." Billy himself appeared in the alley. Reassured at the sight of the youth, but angry, the instructor moved into the alley. "What is the meaning of this? You have alarmed all of us." "I'm awfully sorry, sir. But I saw something back here that interested me, and I stopped to take a look. I hope you will forgive me." His manner was so contrite and his chagrin so complete that the instructor had no choice but to forgive him. "Of course, Billy. But you mustn't do anything like this again. It might be dangerous." "I won't, sir. I promise. But I wonder, since you are here, if you would be good enough to explain to me the thing I saw back here. It will only take a minute." "What is it?" "It's something in one of the houses. I came back looking at something else, then caught a glimpse of this. If you will come into the back yard you can see it. I would really like to have you explain it to me, sir. You are always so clear in your explanations." Billy Kasker's manner was very winning.