Anna glided to a branch and settled there gracefully, folding her wings behind her. Maura killed her engine and, with a soft creak, the air car settled into place in a crook of the branch.

Maura saw some of the other children, seemingly far below. There were two groups, each of four or five kids; the oldest of them looked around ten. After five years on the Moon, they looked skinny, graceful. One group was playing what looked like a tag game, chasing with great loping strides and somersaults and spectacular lunar leaps. Maura could hear them laughing, the sound drifting up to her like the ripple of water.

The other group seemed more solemn. They were moving around each other, but in a series of patterns, each of which they would hold for a fraction of a second of stillness, and then move on to the next. They seemed to be talking, or maybe singing, but Maura couldn’t make out any words.

“Anna, where are the Tybee children? Tom and Billie—”

Anna pointed.

The Tybees were part of the solemn party below. Maura recognized Tom, ten years old now, his face round and set and serious. At his waist he had his electronic Heart — battered, dirty, probably nonfunctioning, a gift from his long-lost mother. She wondered which one of the younger kids was Billie.

Once she had promised his father that she would protect Tom. It was a promise that had brought her all this way. And yet, what protection could she offer him? What could she ever have given him?

“Can you tell me what they are doing down there?”

“They’re working. It’s what your people call—”

“Multiplexing. Yes, I know. What are they talking about?”

Anna’s face worked. “They are considering constraints on the ultimate manifold.”

Maura suspected that she was going to struggle with the rest of this conversation. “The manifold of what?”

“Universes. It is of course a truism that all logically possible universes must exist. The universe, this universe, is described — umm, that’s the wrong word — by a formal system. Mathematics. A system of mathematics.”

Maura frowned. “You mean a Theory of Everything?”

Anna waved a hand, as if that were utterly trivial, and her beautiful wings rustled. “But there are many formal systems. Some of them are less rich, some more. But each formal system, logically consistent internally, describes a possible universe, which therefore exists.”

Maura tried to follow that. “Give me an example of a formal system.”

“The rules of geometry. I mean, Euclid’s geometry.”

“High school stuff.”

Anna looked at her with reproof. “I never went to high school, Maura.”

“I’m sorry.”

“Some of these universes, as described by the formal systems, are rich enough to support self-aware substructures. Life. Intelligence. And some of the universes aren ‘t rich enough. A universe described by Euclidean geometry probably isn’t, for example. Therefore it can’t be observed. What the group down there is trying to establish is whether a universe that cannot be observed, though it exists, may be said to have a different category of existence.” Anna glanced at Maura. “Do you understand?”

“Not a damn word.”

Anna smiled.

Maura could see firefly robots hovering over the heads of the children, peering down, recording everything they did and said. There might be a rich treasure of knowledge and wisdom being conjured up in the dance of those slim forms, but the world’s massed experts couldn’t begin to decode it. IBM had quoted development times in decades just to construct a translation software suite.

The children had, it seemed, evolved their own language from elements of their native spoken languages, mixed with gestures, dance, and music. It was a complex, multilevel communication channel, with many streams of information multiplexed together. Linguists believed it was a true language, with a unifying grammar. But it transcended human languages in the richness of its structure, the speed and compression of its data transmission, the fact that it was analog — the angle of an arm or head held just so seemed to make an immense difference to meaning — and its rate of evolution, sometimes changing daily.

And besides, there seemed to be some features that could not be translated into English, even in principle. Such as new tenses. There was one based on palindromic constructions, symmetric in time, that seemed to be designed to describe situations with looping causality, or even causality violation.

Grammar for a time traveler.

Some theorists were saying that the orderly linear perception of time, of neat cause and effect, enjoyed by humans was an artifact of a limited consciousness: like the way the brain could “construct” an image of a face from a few lines on a page. Perhaps the children could experience time on a deeper level: non-linearly, even acausally.

And the farthest-out theorists wondered if their minds were somehow linked, permanently, by the neutrino ocean that filled the universe. As if Feynman radio technology was allowing some higher strata of consciousness and self-awareness to operate here.

The various strategies that had been tried to keep a handle on the children had yet to pay off. The Trojan Horse kids — like little Billie Tybee, below — seemed to have melted into the strange community here without a backward glance. The Trojan Horses had been heavily indoctrinated with a basic common grammar and quantification rules in the hope that they would at least continue to talk comprehensibly to the outside world. But even that had failed. They just didn’t have the patience or inclination to translate their thoughts into baby talk for their parents.

The only Blue who would regularly talk to those outside was Anna, five or six years older than any of the rest. And the specialist observers believed that — though Anna was the de facto leader of the children here — she was too old, her grammatical sense frozen too early, to have become fully immersed in the complex interchanges that dominated the lives of the rest of the children.

And besides, Anna was hardly a useful ambassador. Adults had damaged her too much.

A section of oak tree trunk seemed to split away, bending stiffly, and a thin, distorted face turned and peered up at Maura.

Maura nearly jumped out of her seat. “Oh, my good gosh.”

Anna laughed.

The giraffe stepped out of the shade of the tree. The yellow-and-black mottled markings on its body had made it almost invisible to Maura, startling for such a huge animal. The giraffe loped easily forward, fine-chiseled head dipping gently, the lunar gravity making no apparent difference to its stately progress. Now two more animals followed the first, another adult and a baby, its neck stubby by comparison.

Anna said, “There are little NASA robot dung beetles that come out at night and roll away their droppings. They’re really funny.”

“Why are they here?”

Anna shrugged. “We asked for them. Somebody saw one in a picture book once.”

Maura watched the giraffes recede, loping easily in the wash of sunlight and crater-wall shadow, their bodies and motion utterly strange, unlike the body plans of any creature she had seen. A real extreme of evolution, she thought.

Just like these damn kids.

Anna’s eyes, gray as moondust, were grave, serious. “Maura, why are you here?”

“You deserve the truth,” Maura said.

“Yes, we do.” Anna looked up at Earth, fat and full, its round-ness slightly distorted by the fabric of the dome. “We see the lights sometimes, on the night side.”

“What do you think they are?”

Anna shrugged. “Cities burning.”

Maura sighed. “Have you studied history, Anna?”

“Yes. The information is limited, the interpretations partial. But it is interesting.”

“Then you’ll know there have been times like this before. The religious wars during the Reformation, for instance. Protestants against Catholics. The Catholics believed that only their priests controlled access to the afterlife. So anybody who tried to deny their powers threatened not just life, but even the afterlife. And the Protestants believed the Catholic priests were false, and would therefore deny their followers access to the afterlife. If you look at it from the protagonists’ point of view, they were reasonable wars to fight, because they were over the afterlife itself.”