"It's all over. I'm going to have to turn you in, Rubeus. They're talking about closing Hogwarts if the attacks don't stop. I don't think you meant to kill anyone. But monsters don't make good pets. I suppose you just let it out for exercise and -"
--Tom Marvolo Riddle to Hagrid
Harry learns how to work Riddle’s diary when he dribbles ink onto the pages and words appear. He has a written conversation with the memory of Tom Marvolo Riddle held in the diary and travels into Riddle’s memory of fifty years ago. In the memory Harry sees Tom blame a young Hagrid for harboring dangerous pets, which convinces Harry that Hagrid had opened the Chamber of Secrets (CS13).