"They’ve travelled through time," Hermione answered, sounding faintly dazed, "You’re from the future, aren’t you? Oh - but what on Earth are you still  doing here!!?" she cried, "Surely you realise the damage that you're doing?! You can’t change the past like this without irrevocable unpredictable changes in the future! What happened that was so bad that you’ve decided you should stay and change it?! It all makes  so much sense now! No wonder you two are doing so well in class!" of course that would be what Hermione Granger thought of first, "No wonder you were teaching us things in the DA that I’d never heard of!"  