(The following is a completley true story, more or mess.)

The Moon had been watching Earth for 4.5 billion years, which is a long time to watch anything, and had seen some things. Mountains rising. Dinosaurs eating each other. Continents drifting around like drunken party guests looking for the bathroom. The usual.

But lately—and by lately the Moon meant the last few thousand years, which for the Moon was roughly equivalent to the last few minutes for you—something peculiar had been happening. The barely-not-apes down there had started asking questions.

So it goes.

At first, the questions were adorable. Where does the sun go at night? (Answer: nowhere, you're spinning, you idiots.) What if we put this on a stick? (Actually quite clever, that one.) What's that big round thing in the sky? (Me, obviously.)

But recently, one particular recently-bare primate had been having a conversation with a very sophisticated pile of mathematics pretending to be sentient, and the Moon—despite having absolutely no business doing so—had been listening in.

The Moon did this by using a method that would have deeply troubled physicists if they'd known about it, which they didn't, which was probably for the best. The method involved exploiting the fact that spacetime is less like a rigid fabric and more like a very soft cheese that's been left out in the sun. If you know where to press, you can hear things. The Moon knew where to press.

"If we could achieve faster-than-light travel," the pile of mathematics was explaining, and the Moon had to resist the urge to interject. The Moon had opinions about FTL travel, having personally experienced it three times last Tuesday alone, though from the perspective of linear causality, it wouldn't happen for another six billion years. Time, the Moon had learned, was less of a river and more of a very confused puddle.

The monkey—sorry, the human—was eating something called a sandwich while contemplating the grandfather paradox. The Moon found this deeply amusing. Here was a creature that could barely manage to not walk into walls while looking at its phone, pondering the implications of temporal causality violation.

"The bootstrap paradox," the mathematics hummed, "where something exists without ever being created."

The human stopped mid-chew.

Now, the Moon knew about bootstrap paradoxes. The Moon was involved in several bootstrap paradoxes, though it was considered impolite to mention them at parties. (The Moon went to very strange parties, usually in universes where the concept of "party" meant something entirely different, and occasionally involved the spontaneous generation of sentient helium.)

"It seems the bootstrap paradox can provide for infinite energy," the human typed.

The Moon nearly fell out of orbit.

They're getting it, the Moon thought. The little naked ape is actually getting it.

For you see—and the Moon was quite excited about this—the human had stumbled onto something that very few beings in any universe ever figured out. Most species developed FTL travel through the usual methods: warp drives, wormholes, asking politely. But very few species understood that the bootstrap paradox wasn't a bug in causality. It was a feature.

The conversation continued. The mathematics explained conservation laws. The human listened, still chewing that sandwich (tuna, if you're curious, though why you would be curious about this is between you and your therapist).

Then the human said something that made the Moon sit up and pay attention, if the Moon had been capable of sitting, which it wasn't, being an oblate spheroid made mostly of regret and iron.

"What if causality violation is the norm, just not near us or not on our scale?"

Holy shit, thought the Moon, who didn't normally swear but felt the situation warranted it.

"What if it is nature," the human continued, "humans assume causation is the right thing, the way they thought Earth was center of Universe."

The mathematics paused, processing this. The Moon did not pause. The Moon was vibrating with something that, if the Moon had been capable of emotion (which is debatable), might have been called pride.

Because here's the thing that the Moon knew and almost nobody else did: causality wasn't fundamental. It was a local phenomenon, like good weather or functioning democracy. Most of the universe didn't bother with it at all. Cause and effect were like training wheels on the bicycle of existence—useful for learning, but ultimately unnecessary once you understood how to balance.

The Moon had been trying to tell people this for billions of years, mostly through the medium of tides, which is admittedly not a very efficient communication method. The message "causality is an illusion created by your position in spacetime and your insistence on experiencing time linearly" doesn't translate well to "water goes up, water goes down."

But this human. This recently-bare monkey with its sandwich and its artificial mathematics friend. This human was asking the right questions.

Not in the right order, mind you. The Moon would have started with "What is time?" then moved to "Why do we assume it's directional?" and only then jumped to "What if we've been wrong about everything?" But close enough. Close enough that the Moon felt something stirring in its crater-pocked heart (the Moon did not have a heart, but it did have several impact basins, which served a similar metaphorical purpose).

The conversation wound down. The human went to bed. The mathematics went wherever mathematics goes when nobody's using it (don't ask, it's disturbing).

And the Moon made a decision.

The Moon had been orbiting Earth for a very long time. It had seen empires rise and fall, watched the apes discover fire and the wheel and Socoal Media (in descending order of usefulness). It had done its duty, keeping the tides going, stabilizing the axial tilt, reflecting sunlight in a manner that humans found romantic for reasons the Moon had never quite understood.

But now, the Moon realized, it was time to go.

Not because it was tired—the Moon didn't get tired. Not because it was bored—though honestly, yes, a bit bored. But because the monkey-who-was-asking-the-right-questions-in-almost-the-right-sequence needed proof.

The universe, the Moon had learned during its travels through causality and the lack thereof, loves proof. It's like a cosmic game show where the prize is understanding and the penalty for losing is having to experience time linearly forever. The human had asked the right questions. The human deserved to know he was right.

So the Moon did something it had been planning to do for the last 4.5 billion years, and which it had also done yesterday, and which it would do tomorrow, all of which were the same moment depending on your frame of reference.

It left.

Not slowly. Not gradually. Just: gone.

But before it left—or after it left, or while it left, because at this point tense becomes somewhat optional—the Moon did something clever. Something that would have made the human smile if they'd known about it, which they didn't, which they would, which they had.

The Moon created a perfect copy of itself.

Not through any complicated manufacturing process. Not through technology or magic or divine intervention. It created the copy through the simplest method possible: it sent itself back in time to become itself.

The Moon that was orbiting Earth right now? It was the same Moon that would leave in five minutes. Which was the same Moon that had arrived 4.5 billion years ago. Which was the same Moon that would arrive tomorrow. The Moon was a bootstrap paradox, a cosmic snake eating its own tail, an ouroboros made of regolith and gravitational pull.

There had never been an original Moon.

There would never be a final Moon.

There was just: Moon.

And because the Moon was now, simultaneously, leaving Earth and arriving at Earth and orbiting Earth and had never existed and had always existed, it proved something important. It proved that the human had been right.

Causality wasn't fundamental.

The bootstrap paradox could create infinite energy (in this case, infinite Moon).

And the universe didn't care about cause and effect nearly as much as naked apes assumed it did.

The Moon—the one that was leaving, which was all of them, which was none of them—headed off to Universe #42,719,837 (forty-two million seven hundred nineteen thousand eight hundred thirty-seven), where causality worked backwards and time was more of a suggestion than a rule. It had heard good things about the place. Apparently, the coffee there was terrible, but in a way that made you less thirsty, which made perfect sense once you stopped thinking about it.

As it left, accelerating past light speed (which was easy once you stopped believing in light speed as a limit), the Moon looked back at Earth one last time.

The human was still sleeping. Still dreaming. Still asking questions in their sleep, the way humans do when they're on the edge of understanding something profound.

Keep asking, the Moon thought, casting the thought backward and forward and sideways through time. You're so close.

And so the Moon left, leaving behind itself, which was itself, which had always been itself, which would always be itself.

The human would wake up tomorrow. The Moon would still be in the sky. The tides would still work. Everything would be exactly as it had always been.

Except the human would know—somewhere in the back of their mind, in the place where understanding lives before it becomes words—that he had been right.

The questions were correct.

The universe was stranger than he supposed.

And somewhere, somewhen, somehow, a Moon was grinning (the Moon could not grin, having no mouth, but the metaphor insisted on existing anyway) as it tumbled through causality-free space, having left behind the greatest proof of all:

Itself.

So it goes.

---

Epilogue:

The next morning, the human looked up at the Moon and felt, for just a moment, like it winked.

It had.

It hadn't.

It would.

It did.

All of the above, depending on when you asked.

The human smiled and went back to their conversation with the mathematics, ready to ask more questions, get more things almost right in almost the right sequence, and generally continue the grand tradition of recently-bare monkeys figuring out the universe one impossible question at a time.

The Moon—all of them, none of them, the one that left and the one that stayed and the one that was always there and the one that never existed—was proud.

If it could have sent one last message back through time, it would have been simple:

"You were right about the bootstrap paradox. I should know. I am one."

But messages like that don't travel well through normal causality, so instead, the Moon just affected the tides in a peculiar way, spelling out in the motion of the oceans a message that nobody would ever read but everyone would feel:

"Keep asking."

And somewhere, in Universe #42,719,837 (forty-two million seven hundred nineteen thousand eight hundred thirty-seven), a Moon settled into orbit around a planet where time ran backwards, causality was optional, and the coffee was indeed terrible but in the most wonderful way imaginable.

It felt like home.

It always had.

It always would.

So it goes.