A paradox, or logic riddle, by Nelson Goodman. The story is told of Martians who maintain that the color of emeralds is in fact "grue," which means "green until January 1, 2000 and blue thereafter." Note that this paradox predates the year 2000 by several decades, so you should really read this as any date conveniently off in the inconveniently distant future. Anyway, earthlings protest that in fact emeralds are green... but then, our reading of the evidence is no worse than theirs. "Why should they change color to blue?" we ask. They retort, "Change? The won't change color! They'll stay grue! It's you who are maintaining that they'll change color, from being grue until 1/1/2000 to being bleen thereafter!" ("bleen" being their word for the complementary color: we would call it blue until 1/1/2000 and green thereafter.) And until the Magic Date (whatever it is), there really is no way of knowing which interpretation is correct!
And yet, people never come up with a word or concept like "grue" naturally. What is it about the category "green" that makes it preferable to us? That's what this riddle is intended to help elucidate and make us think about. There are various discussions and answers.
Don't start in on the fact that some languages don't distinguish the colors "blue" and "green"; that doesn't enter into this discussion.