At the end of the book, you find out that Jim was free all along (Miss Watson freed him after she died). What was Twain's purpose in doing that? I thought it may have just been farcical because Huckleberry Finn is largely satire. Is there like a deeper significance to it?

Your teacher doesn't want to know what I think. Is there some irony in this ending? Was the effort to escape slavery worth it if he was going to be freed anyway? How would Jim and Huck have known that Miss Watson was going to free him in her will? Was Clemens trying to say the troubles Jim and Huck went through were wasted, or was he trying to bring his story to a happy ending with Jim no longer having to worry about a return to or continued slavery? Think about it. Maybe you have some other questions to bring to bear on the question of what Twain/Clemens' purpose was?