As evidenced by his disdain for doors, Tom is one of those people who likes to make life a little harder than it should be.
Because of this tendency, Tom devises lots of weird, literary-based strategies to help Jim escape.
First he wants a saw to take off the leg from Jim's bed (that's where he's chained up).
Huck cleverly remarks that you could simply lift up the bed, but like doors, such actions are for suckers.
Then he wants to actually saw Jim's leg off. And make a moat around the cabin. And bake Jim a pie with a rope ladder hidden inside.
All of this is necessary, he insists, since that's how they do it in adventure books.
Huck steals some supplies (shirt, sheet) from the Phelpses. He tries to call it "borrowing," but Tom, who apparently is now all morals, tells him that, in fact, it's stealing, so let's just call a horse a horse.
However, he also says that the stealing is okay, since everyone knows prisoners get a free pass to steal what they need.