Why is Aunt Polly so angry with Tom after she talks with Sereny Harper about Tom's dream?

Aunt Polly was so angry with Tom because he had lied about "the prophecy", his dream where he saw them in his room and told them everything they had done as if he was there in spirit.

Explanation:

The Adventures of Tom Sawyer by Mark Twain tells the story of a young boy named Tom Sawyer and his experiences, looking for adventure. It also deals with the mischievousness of the main character, who lives with his aunt and half brother.

Chapter 18 shows the scene where Tom told his Aunt Polly about the dream he had seen of them worrying about him in his room, his bed, mourning. He had also seen "Joe Harper's mother", which was exactly true. he then began telling them everything he had seen them do that night. This made them believe that Tom had actually had a vision and seen them even though he wasn't really present at that time.

But Chapter 19 showed Tom being scolded by Aunt Polly because she had realized that whatever Tom had told about the "vision" was all a lie. Sereny Harper had told her that Joe had seen Tom watching them that night and had actually been there too, but faked the prophecy so as to escape the brunt of his Aunt's anger.