Chapter 77:

She and Harry had looked up Divination early on in their research; Harry had insisted that they read everything they could find about prophecies that wasn't in the Restricted Section. As Harry had observed, it would save a lot of effort if they could just get a seer to prophesy everything they would figure out thirty-five years later. (Or to put it in Harry's terms, any means of obtaining information transmitted from the distant future was potentially an instant global victory condition.)

But, as Hermione had explained to Millicent, prophesying wasn't controllable, there was no way to ask for a prophecy about anything in particular. Instead (the books had said) there was a sort of pressure that built up in Time, when some huge event was trying to happen, or stop itself from happening. And seers were like weak points that let out the pressure, when the right listener was nearby. So prophecies were only about big, important things, because only that generated enough pressure; and you almost never got more than one seer saying the same thing, because afterward the pressure was gone. And, as Hermione had further explained to Millicent, the seers themselves didn't remember their prophecies, because the message wasn't for them. And the messages would come out in riddles, and only someone who heard the prophecy in the seer's original voice would hear all the meaning that was in the riddle. There was no possible way that Millicent could just give out a prophecy any time she wanted, about school bullies, and then remember it, and if she had it would've come out as 'the skeleton is the key' and not 'Susan Bones has to be there'.