“That’s Nanny,” Bobby said.

And into the room came Nanny.

Mr. Fields watched her. She had always intrigued him. The only sound in the room was her metal treads, scraping against the hardwood floor, a peculiar rhythmic sound. Nanny came to a halt in front of him, stopping a few feet away. Two unwinking photocell eyes appraised him, eyes on flexible wire stalks. The stalks moved speculatively, weaving slightly. Then they withdrew.

Nanny was built in the shape of a sphere, a large metal sphere, flattened on the bottom. Her surface had been sprayed with a dull green enamel, which had become chipped and gouged through wear. There was not much visible in addition to the eye stalks. The treads could not be seen. On each side of the hull was the outline of a door. From these the magnetic grapples came, when they were needed. The front of the hull came to a point, and there the metal was reinforced. The extra plates welded both fore and aft made her look almost like a weapon of war. A tank of some land. Or a ship, a rounded metal ship that had come up on land. Or like an insect. A sowbug, as they are called.

“Come on!” Bobby shouted.

Abruptly Nanny moved, spinning slightly as her treads gripped the floor and turned her around. One of her side doors opened. A long metal rod shot out. Playfully, Nanny caught Bobby’s arm with her grapple and drew him to her. She perched him on her back. Bobby’s legs straddled the metal hull. He kicked with his heels excitedly, jumping up and down.

“Race you around the block!” Jean shouted. “Giddup!” Bobby cried. Nanny moved away, out of the room with him. A great round bug of whirring metal and relays, clicking photocells and tubes. Jean ran beside her. There was silence. The parents were alone again. “Isn’t she amazing?” Mrs. Fields said. “Of course, robots are a common sight these days. Certainly more so than a few years ago. You see them everywhere you go, behind counters in stores, driving buses, digging ditches—”

“But Nanny is different,” Tom Fields murmured.

“She’s—she’s not like a machine. She’s like a person. A living person. But after all, she’s much more complex than any other kind. She has to be. They say she’s even more intricate than the kitchen.”

“We certainly paid enough for her,” Tom said.

“Yes,” Mary Fields murmured. “She’s very much like a living creature.” There was a strange note in her voice. “Very much so.”

“She sure takes care of the kids,” Tom said, returning to his newspaper.

“But I’m worried.” Mary put her coffee cup down, frowning. They were eating dinner. It was late. The two children had been sent up to bed. Mary touched her mouth with her napkin. “Tom, I’m worried. I wish you’d listen to me.”

Tom Fields blinked. “Worried? What about?”

“About her. About Nanny.”

“Why?”

“I—I don’t know.”

“You mean we’re going to have to repair her again? We just got through fixing her. What is it this time? If those kids didn’t get her to—”

“It’s not that.”

“What, then?”

For a long time his wife did not answer. Abruptly she got up from the table and walked across the room to the stairs. She peered up, staring into the darkness. Tom watched her, puzzled.

“What’s the matter?”

“I want to be sure she can’t hear us.”

“She? Nanny?”

Mary came toward him. “Tom, I woke up last night again. Because of the sounds. I heard them again, the same sounds, the sounds I heard before. And you told me it didn’t mean anything!”

Tom gestured. “It doesn’t. What does it mean?”

“I don’t know. That’s what worries me. But after we’re all asleep she comes downstairs. She leaves their room. She slips down the stairs as quietly as she can, as soon as she’s sure we’re all asleep.”

“But why?”

“I don’t know! Last night I heard her going down, slithering down the stairs, as quiet as a mouse. I heard her moving around down here. And then—”

“Then what?”

“Tom, then I heard her go out the back door. Out, outside the house. She went into the back yard. That was all I heard for awhile.”

Tom rubbed his jaw. “Go on.”

“I listened. I sat up in bed. You were asleep, of course. Sound asleep. No use trying to wake you. I got up and went to the window. I lifted the shade and looked out. She was out there, out in the back yard.”

“What was she doing?”

“I don’t know.” Mary Fields’s face was lined with worry. “I don’t know! What in the world would a Nanny be doing outside at night, in the back yard?”

It was dark. Terribly dark. But the infrared filter clicked into place, and the darkness vanished. The metal shape moved forward, easing through the kitchen, its treads half-retracted for greatest quiet. It came to the back door and halted, listening.

There was no sound. The house was still. They were all asleep upstairs. Sound asleep.

The Nanny pushed, and the back door opened. It moved out onto the porch, letting the door close gently behind it. The night air was thin and cold. And full of smells, all the strange, tingling smells of the night, when spring has begun to change into summer, when the ground is still moist and the hot July sun has not had a chance to kill all the little growing things.

The Nanny went down the steps, onto the cement path. Then it moved cautiously onto the lawn, the wet blades of grass slapping its sides. After a time it stopped, rising up on its back treads. Its front part jutted up into the air. Its eye stalks stretched, rigid and taut, waving very slightly. Then it settled back down and continued its motion forward.

It was just going around the peach tree, coming back toward the house, when the noise came.

It stopped instantly, alert. Its side doors fell away and its grapples ran out their full lengths, lithe and wary. On the other side of the board fence, beyond the row of shasta daisies, something had stirred. The Nanny peered, clicking filters rapidly. Only a few faint stars winked in the sky overhead. But it saw, and that was enough.

On the other side of the fence a second Nanny was moving, making its way softly through the flowers, coming toward the fence. It was trying to make as little noise as possible. Both Nannies stopped, suddenly unmoving, regarding each other-the green Nanny waiting in its own yard, the blue prowler that had been coming toward the fence.

The blue prowler was a larger Nanny, built to manage two young boys. Its sides were dented and warped from use, but its grapples were still strong and powerful. In addition to the usual reinforced plates across its nose there was a gouge of tough steel, a jutting jaw that was already sliding into position, ready and able.

Mecho-Products, its manufacturer, had lavished attention on this jaw-construction. It was their trademark, their unique feature. Their ads, their brochures, stressed the massive frontal scoop mounted on all their models. And there was an optional assist: a cutting edge, power-driven, that at extra cost could easily be installed in their “Luxury-line” models.

This blue Nanny was so equipped.

Moving cautiously ahead, the blue Nanny reached the fence. It stopped and carefully inspected the boards. They were thin and rotted, put up a long time ago. It pushed its hard head against the wood. The fence gave, splintering and ripping. At once the green Nanny rose on its back treads, its grapples leaping out. A fierce joy filled it, a bursting excitement. The wild frenzy of battle.

The two closed, rolling silently on the ground, their grapples locked. Neither made any noise, the blue Mecho-Products Nanny nor the smaller, lighter, pale-green Service Industries, Inc., Nanny. On and on they fought, hugged tightly together, the great jaw trying to push underneath, into the soft treads. And the green Nanny trying to hook its metal point into the eyes that gleamed fitfully against its side. The green Nanny had the disadvantage of being a medium-priced model; it was outclassed and outweighed. But it fought grimly, furiously.