"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."

"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.

Sudd