I once took a critical thinking course and bought the textbook 2nd hand from the college bookstore. A week or two in, I noticed half of a sentence written in the margin. As the professor started teaching the topic from that page, he rhetorically asked a question, did not get an answer, and then answered himself with the sentence from the book. I filliped ahead and found that the entire book was annotated with all of…
From the wiki page: `Harry finds an old potions textbook, once belonging to "The Half-Blood Prince", an individual who wrote numerous spells and tips in the book. Harry, using the tips in the book, excels in Potions class, winning a bottle of Felix Felicis, or "Liquid Luck".`