Kid on a raft, bad guys, several snake-related incidences—you're just one Samuel L. Jackson (and a few technological innovations) away from Snakes on a Plane. (Although, to be fair, Twain is also clearly drawing from classic adventure epics, particularly The Odyssey. Don't believe us? Well, consider that, throughout the whole epic, the main character is called the "wily" or "crafty" Odysseus. Translation: the guy is really good at spinning a story—just like our friend Huck.)
But there's clearly something else going on here. For one, take Twain's "Notice": "Persons attempting to find a motive in this narrative will be prosecuted; persons attempting to find a moral in it will be banished; persons attempting to find a plot in it will be shot."
Um, satirical, much? By reading this first, we know that we should pay close attention: this may look like a kid's book, like one of the adventures rotting Tom Sawyer's brain, but everything is not as it seems. It may look like a kid's book, but in fact it's a complicated examination into racism, slavery, and the moral issues that go with them.
And speaking of morality: Huck doesn't exactly grow up over the course of his travels, but he does develop his moral compass to a significant degree, which is a big part of becoming an "adult." Sure, he's not about to settle down with a nice girl and a picket fence—but he's definitely growing up.