They trade a live tick Huck found in the woods for Tom’s tooth.
At the beginning of chapter 6, Tom loses a tooth. Aunt Polly had yanked it out that morning. Huck has a dead cat he plans to use to remove warts at a local cemetery. He also seems to have located a tick, still alive. Tom suggests that tick might be a bit small, but Huck is convinced of its merits.
“Oh, anybody can run a tick down that don't belong to them. I'm satisfied with it. It's a good enough tick for me.” (chapter 6, enotes etext p. 34)
Huck comments that it is early in tick season, and Tom shows him his tooth. Tom shows Huck the hole in his mouth to prove that the tooth is “genuwyne” and it’s a fair trade.
As a result of this trade, Tom is late for school and gets whipped by the schoolmaster. He has to sit with the girls, and he’s not bothered at bit!