There was a sound.

“Mommy!” Jean and Bobby came scampering into the living room, their faces flushed with pleasure. “Mommy, we raced Nanny all the way home, and we won!”

“We won,” Bobby said. “We beat her.”

“We ran a lot faster than she did,” Jean said.

“Where is Nanny, children?” Mrs. Fields asked.

“She’s coming. Hello, Daddy.”

“Hello, kids,” Tom Fields said. He cocked his head to one side, listening. From the front porch came an odd scraping sound, an unusual whirr and scrape. He smiled.

“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.