While the Gryffindors were plotting something big, Andrew was in the office trying his hand at grading the Fourth Years' homework.
He had taken the job to learn from Professor McGonagall, but after several days of grading, he found the homework taught him as much as spending the same amount of time reading in the library.
Whether he was reading, practicing on his own, or learning from Professor McGonagall, he was always shown the right way: the right technique, the right incantation, the right approach, the right conclusion.
Having no mistakes was good, but not entirely. He had no idea where people were likely to go wrong, or what different mistakes would lead to.
It was an imperfect comparison, but take Professor Snape's classes: Andrew could brew a perfect potion, yet when he saw a failed batch, he could only tell that something was wrong. He couldn't glance at it like the professor and identify exactly which step this student—where had he left his brain, anyway?—had botched so spectacularly that it deserved a point off.
That had changed. First- and Second Years' work hadn't given him that feeling, but Third Years' work had caught his attention at once. He finally realized Transfiguration could go wrong in ways he had never imagined!
Once he realized that, he took advantage of his time as a teaching assistant to start looking through the Fourth Years' homework. He couldn't perform that level of Transfiguration yet, but he understood quite a bit of the theory.
After a quick look at the students' previous grades, he found the assignments of the top three, whose theoretical mistakes were virtually nonexistent. Using their work as a model, he began working backward to study everyone else's errors.
"You can even do it like that?"
"Hmm, that mistake... I was