In chapter 6 of Tom Sawyer, the titular Tom comes across the town ne’er-do-well, Huckleberry Finn. Huck is carrying a dead cat with him on the way to the graveyard. The superstition is that dead cats are good for curing warts. After a wicked person has been buried and the devil comes to take away their corpse, the dead cat will follow the corpse, and the warts will follow the cat. Or, as Huck Finn tells it:
Why, you take your cat and go and get in the graveyard 'long about midnight when somebody that was wicked has been buried; and when it's midnight a devil will come, or maybe two or three, but you can't see 'em, you can only hear something like the wind, or maybe hear 'em talk; and when they're taking that feller away, you heave your cat after 'em and say, 'Devil follow corpse, cat follow devil, warts follow cat, I'm done with ye!' That'll fetch any wart.
Twain included this explanation of the cat as a memorable way to introduce Huck Finn and probably as a way to poke fun at the superstitious beliefs so prevalent during his time. The fact that Huck Finn believes in these superstitions tells the reader a lot about his upbringing and worldview. It is also telling that Huck has procured a dead cat in the first place; it shows the reader what a hooligan he is right off the bat.