The slope was a gentle one and even the weight of the spacesuit couldn't keep Jimmy from racing up it in a floating hop that made the gravity seem nonexistent.
"Come on, Robutt," he shouted.
Robutt, who could hear him by radio, squeaked and bounded after.
Jimmy, expert though he was, couldn't outrace Robutt, who didn't need a spacesuit, and had four legs and tendons of steel. Robutt sailed over Jimmy's head, somersaulting and landing almost under his feet.
"Don't show off, Robutt," said Jimmy, "and stay in sight."
Robutt squeaked again, the special squeak that meant "Yes."
"I don't trust you, you faker," shouted Jimmy, and up he went in one last bound that carried him over the curved upper edge of the crater wall and down onto the inner slope.
The Earth sank below the top of the crater wall and at once it was pitch-dark around him. A warm, friendly darkness that wiped out the difference between ground and sky except for the glitter of stars.
Actually, Jimmy wasn't supposed to exercise along the dark side of the crater wall. The grown ups said it was dangerous, but that was because they were never there. The ground was smooth and crunchy and Jimmy knew the exact location of every one of the few rocks.
Besides, how could it be dangerous racing through the dark when Robutt was right there with him, bouncing around and squeaking and glowing? Even without the glow, Robutt could tell where he was, and where Jimmy was, by radar. Jimmy couldn't go wrong while Robutt was around, tripping him when he was too near a rock, or jumping on him to show how much he loved him, or circling around and squeaking low and scared when Jimmy hid behind a rock, when all the time Robutt knew well enough where he was. Once Jimmy had lain still and pretended he was hurt and Robutt had sounded the radio alarm and people from Lunar City got there in a hurry. Jimmy's father had let him hear about that little trick, and Jimmy never tried it again.
Just as he was remembering that, he heard his father's voice on his private wavelength. "Jimmy, come back. I have something to tell you."
Jimmy was out of his spacesuit now and washed up. You always had to wash up after coming in from outside. Even Robutt had to be sprayed, but he loved it. He stood there on all fours, his little foot-long body quivering and glowing just a tiny bit, and his small head, with no mouth, with two large glassed-in eyes, and with a bump where the brain was. He squeaked until Mr. Anderson said, "Quiet, Robutt."
Mr. Anderson was smiling. "We have something for you, Jimmy. It's at the rocket station now, but we'll have it tomorrow after all the tests are over. I thought I'd tell you now."
"From Earth, Dad?" "A dog from Earth, son. A real dog. A Scotch terrier puppy. The first dog on the Moon. You won't need Robutt any more. We can't keep them both, you know, and some other boy or girl will have Robutt." He seemed to be waiting for Jimmy to say something, then he said, "You know what a dog is, Jimmy. It's the real thing. Robutt's only a mechanical imitation, a robot-mutt. That's how he got his name."
Jimmy frowned. "Robutt isn't an imitation, Dad. He's my dog." "Not a real one, Jimmy. Robutt's just steel and wiring and a simple positronic brain. It's not alive."
"He does everything I want him to do, Dad. He understands me. Sure, he's alive."
"No, son. Robutt is just a machine. It's just programmed to act the way it does. A dog is alive. You won't want Robutt after you have the dog."
"The dog will need a spacesuit, won't he?" "Yes, of course. But it will be worth the money and he'll get used to it. And he won't need one in the City. You'll see the difference once he gets here."
Jimmy looked at Robutt, who was squeaking again, a very low, slow squeak, that seemed frightened. Jimmy held out his arms and Robutt was in them in one bound. Jimmy said, "What will the difference be between Robutt and the dog?"
"It's hard to explain," said Mr. Anderson, "but it will be easy to see. The dog will really love you. Robutt is just adjusted to act as though it loves you."
"But, Dad, we don't know what's inside the dog, or what his feelings are. Maybe it's just acting,