Robin told him about the legend of a people who lived in a mythical world. Their oceans were small and timid compared to the Grand Line, and the seas were divided not because of their traits or geographical locations, but through the separation of land and governments. They didn't have a World Government, and their islands were called 'continents' – land masses that were as large as the country of Alabasta or even bigger.
Luffy propped his chin up on the table, the rest of his body dangling off the edge. His legs swung lazily back and forth, rocking with the motion of the ship shifting on the waves.
She didn't tell stories like Ussop, with frantic arm-waving and dramatic poses. She didn't stand up on the chair with a foot planted firmly on the tabletop – voice ringing against one's ears, proclaiming tales of heroism and bravery to the silent listening seas.
She read from books; the crinkle of a page turning and a low chuckle. She showed him whatever pictures she came across (to Luffy's disappointment, there weren't many).
The people of the legend had believed that their world was flat (Luffy wondered out loud, "Like a piece of paper?" and Robin smiled) and that many feared they would fall off the edge if they ventured too far out from land.
Luffy protested. He wanted to fall off the edge of something too. It would be like Skypeia all over again but without knowing if the ocean waited below. It would be an adventure!
He pointed out the moon through the window. In the cloudless sky it shimmered with a hint of silver, swollen and full – shallow against the darkness of the night.
Luffy said that if those people thought the world was flat, then maybe they thought the moon was too. He wanted to fall off the moon because it was so high up and they might see One Piece from there. Robin, that'd be great, right?
Robin agreed.