"Twilight," Princess Celestia said. "What did I just say?"
Twilight Sparkle blinked, and glanced at the books scattered about the reading desk between her and the princess, then at the odd collection of books and strange devices in the lobby of the royal library beyond, desperately seeking some cue.
"If you don't know," the princess said patiently, "then the correct answer is, 'I don't know.'"
"Well... I..." Twilight hung her head. "Don't know," she finished.
"See? That wasn't so hard. Now let's try a harder one: What's on your mind?"
Twilight blushed, and took an involuntary step back. "Nothing!"
"Nothing? Just standing there, empty head, not a care in the world? Twilight, you couldn't have nothing on your mind if you tried. Which you will try, by the way. Remind me to give you some sutras by Maregarjuna for your trip back."
"Maregarjuna?"
"No. First, Twilight Sparkle."
Twilight wrinkled her nose in the tiniest possible display of resistance. "It... isn't my place to question you."
"Oh," Celestia said, "this should be good." She put one foreknee on the table, turned the foreleg upward, propped her head on a hoof, and cocked her head at Twilight, waiting.
"I was thinking about Luna." She looked to Celestia for some reaction, but the princess merely kept smiling.
Eventually, Twilight continued. "All those years on the moon... Centuries. There's no sound there. No color. No smell. Nothing but dust and rocks. It would drive a pony insane."
"Go on."
"Well... it's just..." Twilight stopped, and this time seemed to have run out of words for good.
"How could I?"
Twilight looked away from Celestia and nodded.
Celestia smiled. "I've been waiting for you to ask that question."