The cubes — or “smart pebbles” — that Gilpin and Rus built to test their algorithm enact the simplified, two-dimensional version of the system. Four faces of each cube are studded with so-called electropermanent magnets, materials that can be magnetized or demagnetized with a single electric pulse. Unlike permanent magnets, they can be turned on and off; unlike electromagnets, they don’t require a constant current to maintain their magnetism. The pebbles use the magnets not only to connect to each other but also to communicate and to share power. Each pebble also has a tiny microprocessor, which can store just 32 kilobytes of program code and has only two kilobytes of working memory.

The pebbles have magnets on only four faces, Gilpin explains, because, with the addition of the microprocessor and circuitry to regulate power, “there just wasn’t room for two more magnets.” But Gilpin and Rus performed computer simulations showing that their algorithm would work with a three-dimensional block of cubes, too, by treating each layer of the block as its own two-dimensional grid. The cubes discarded from the final shape would simply disconnect from the cubes above and below them as well as those next to them.

Science fiction fans have been given ideas like this to think about for a long time. In his 1954 novel The Invincible, Stanislaw Lem writes about a shape-forming swarm of tiny metal particles.

Update: In his 1920 novel The Metal Monster, Abraham Merritt envisions this same idea, describing living metal cubes that work together to form different strutures "as though a child should build from nursery blocks a fantastic shape which abruptly is filled with throbbing life.":

"...They were such LITTLE THINGS," muttered Drake. "Such little things—bits of metal—little globes and pyramids and cubes—just little THINGS..."

"Bits of metal"—Dick's gaze sought mine, held it—"and they looked for each other, they worked with each other—THINKINGLY, CONSCIOUSLY—they were deliberate, purposeful—little things—and with the force of a score of dynamos—living, THINKING—"

Thanks to Blue Monkey for contributing the tip on this item. End update.