Why does Tom think he knows what he is talking about when it comes to warts?
In Chapter 6, Tom and Huck discuss the best way to cure warts. While Huck thinks that cats are the best cure for warts, Tom disagrees. He thinks that using spunk-water (rain water which collects in a rotten stump) in a specific ritual will get rid of warts more effectively.
...just as it's midnight you back up against the stump and jam your hand in and say: 'Barley-corn, barley-corn, injun-meal shorts, Spunk-water, spunk-water, swaller these warts,' and then walk away quick, eleven steps, with your eyes shut, and then turn around three times and walk home without speaking to anybody. Because if you speak the charm's busted.
Even though Huck tells Tom that Bob Tanner knows how to use spunk-water to get rid of warts, Tom says that Bob's way with spunk-water is just plain ineffective because Bob is still the 'wartiest boy in town.' This is why Tom thinks he knows better than Huck when it comes to warts.
The two boys argue with each other the way young boys usually do: this competitiveness fuels camaraderie and a sense of rapport between the boys.
Thanks for the question!