The story is "Universe." (Note that science fiction maintains an obsessively detailed classification system for stories, worthy of Linnaeus himself, based on length. A given work of fiction can be a novella, a novelette, a short story or a short-short. For the purposes of my blog, I hereby arbitrarily declare anything that's not book-length to be a short story).
In "Universe," young Hugh Hoyland sets out on a voyage of discovery across his world, which the inhabitants call "The Ship." Everyone living in the ship realizes that the old stories and poems about the "Trip to Far Centaurus" are simply an allegory about the journey through life. After all, the Ship is all that exists, so how could it move through anything? But Hugh eventually discovers the truth: the world in which he lives really is a ship, and it really is travelling between the stars. Heinlein had come up with his own solution to the problem of interstellar travel: instead of trying to evade the speed of light, he decided to live within its limits by inventing the generation ship. If travel to the nearest star would take thousands of years, the ship itself could become a colony, with generations of people living and dying on it before the ship reached its destination. And more likely than not, the inhabitants of the generation ship would eventually forget the original purpose of the ship and mistake their vessel for the entire universe.
The idea of an amnesiac generation ship has since become a staple of both print and screen science fiction -- you might have seen the original Star Trek episode "For the World is Hollow and I Have Touched the Sky". But surely there ought to be some way for the inhabitants of a generation ship to preserve their memories through the millenia. What about books or film? And yet our own history shows how fragile these can be. Consider the thousands of ancient Greek and Roman manuscripts that are forever lost to us. Or even worse, the 97 episodes of Dr. Who destroyed by the BBC.
Indeed, the problem of preserving memories for millenia has become an issue for contemporary scientists on Earth – how can we mark nuclear waste sites so that future generations will avoid them if all memory of civilization were to fail? The answer that the scientists proposed seems paradoxical: the best solution is the Stone-Age solution. The scientists suggested using pictures (not words!) carved into granite. This sounds ridiculous, but remember that ancient clay tablets allow us to reconstruct life 4000 years ago in Mesopotamia, while you would have serious trouble reading a 1980s-era floppy disk. Technology is both fragile and rapidly outdated – rocks are not. So our space travelers might want to bring along some stone tablets, engraved with pictures of their home planet.
One place where this idea is taken seriously is the Museum of History in Granite in Felicity, California, where the history of civilization is preserved on hundreds of granite panels. You may laugh, but it will survive us all.