“We’d like to but we can’t,” Marce said, answering anyway. “The Flow isn’t something that we control, and really, if we’re honest with ourselves, it’s something that we don’t understand very well. It’s like a natural feature of the universe. We can access it but we can’t really do anything with it but go where it’s going anyway. In fact, that makes for one of the really unusual features about the Interdependency.”

Marce zoomed out, wiped the star field, and put up a grid of the forty-seven systems of the Interdependency. The systems featured stars ranging from red dwarfs to sun-like yellow stars, harboring anywhere between one and a dozen major planets. The images of the systems were not to scale and showed the planets zooming along in their orbits, some so quickly it was comical. A few of the children laughed.

“Humans live in all of these star systems, but the planets in most of these star systems aren’t the kinds that are good for human life.” Marce zoomed into Hub again. “Hub, for example, is airless and tidally locked. That means one side of the planet is always facing the sun, so it’s super-hot, and the other side is facing away, which makes it frozen. Humans on Hub have to live under the ground to survive.”

He zoomed out and picked another system. “Here in the Morobe system, the only planets are gas giants—huge planets that don’t even have a surface to land on. We couldn’t live on those. These planets have moons but most of them aren’t very suitable for humans either. So here we live in space habitats, positioned in spots called Lagrange points, or in other places that can be made stable. So that’s how most humans live now: underground on rocky planets or on large space habitats. There’s only one place in the Interdependency where humans live on the surface of the planet.”

Marce zoomed out and in again, back to End, which hovered on the screen, a blue-green marble, clouded in white. “That’s us. That’s End.”

“What about Earth?” one of the children asked, as one of them always did.

“Good question!” Marce said. “Earth is where humans originally come from, and like on End, you could walk around on its surface. But the Earth isn’t part of the Interdependency. We lost contact with the Earth over a thousand years ago when the single Flow stream to it disappeared.”

“How did that happen?” It was the adult again, who was immediately shushed by one of the other adults. Marce smiled at this.

“It’s complicated,” he said. “The best nontechnical answer I can give you is that everything in the universe is constantly moving, including star systems, and that movement sometimes affects the Flow. Basically, the Earth moved, we moved, and the Flow stream went away.”

“Can it happen again?”