A classic is a piece of work which remains appreciated and lauded over a long period of time. It has been almost two centuries since The Adventures of Huckleberry Finn saw its initial publication, yet it remains not only studied, but enjoyed all this time later.
The things which keeps Huckleberry Finn entertaining are its characters and sense of adventure. Huck's first-person narration is rendered in a colorful, engaging style, with plenty of humor to make the boy likable in spite of his flaws. His rebelliousness against civilization is endearing. Furthermore, Huck himself is a three-dimensional character, crude and fun-loving, yet also deeply moral, considering his conflict between wanting to help Jim escape and breaking the law by doing so. The misadventures Huckleberry finds himself in are comical and fun, which make the novel easier to appreciate for younger readers.
The novel's social and moral themes also keep it evergreen. Huck is in danger of becoming like his brutal, selfish, racist father, but his encounter with Jim makes him question everything he has been taught about morality (particularly religious morality, which at this time does not condemn slavery as wrong) and who is worthy of human dignity. Through these thematic conflicts, Huck comes of age and makes his own way in the world. He does not become his father, but neither does he become "sivilized" as the Widow Douglas would have him be. At the end of the story, Huck opts to go west and continue having adventures, living life on his own terms.
The combination of memorable characters, lively adventure, social/moral questions, and a fierce individualist streak have kept Huckleberry Finn relevant and engaging, even by twenty-first-century standards. As American literature goes, it is quintessentially American in the virtues of its hero (individualism, adventurousness) and its questions about society's values.