He pretends to want to write a biography of the man. Writes all about what an admirer he is of Dumbledore, how he looked up to him during his days at Hogwarts, and exalting all the things the headmaster taught him, and so on, and how he wishes more people could know this kind, gentle man for who he truly is, behind all that fame and reputation and all that. Of course, you mustn’t tell Dumbledore! No, that would ruin the surprise! He might have missed Dumbledore’s one-hundredth birthday, but surely, he can have a wonderful biography ready and published in time for his one-hundred-and-eleventh birthday instead, if only he has enough material to write on.