Harry had homework for all his classes. Surprisingly, Snape's defense class always seemed to be the lightest burden. Harry knew it wasn't because the assignments were any less difficult. Harry just liked applications.
With that thought in mind, he'd holed up working through some poisons and antidotes for his Potions class. It helped if he imagined using them on a (younger) Malfoy. You didn't need to kill the bloke to make his life miserable...
Hopefully, none of the resident mindreaders would see into Harry's mind, and conclude he was some sort of sadistic demon-monkey. It was fantasy. He wouldn't actually do it, even if he could be transported back in time.
But it kept him focused. Which turned out to be an unfortunate thing, as Professor McGonagall came into the Common Room to obtain her apprentice. "Did you forget your appointment with me?"
"yes, Professor," Harry said, trying for contrition, but knowing that he'd failed. It was hard to lie.
Professor McGonagall led Harry to her office, that was appointed like a plush battlefield chamber of war. Cushions, yes, but the primary concern was efficiency. It wasn't arranged like Flitwick or Sprout's office. Those said Teacher. This office said Battlefield Wisdom.
Harry settled himself in one of the more penitent seats, feeling more at home than he'd ever felt in Sprout's dirty office.*
Professor McGonagall asks him, "What did you think of the third principle of Wentwood and Wordsworth?"
Harry blinked at her, "I hadn't gotten that far, actually."
Professor McGonagall sighs, "You were just studying potions. Do you think the use of newly drawn wellwater is essential to the Animagus Potion?"
Harry shook his head, "No?"
Professor McGonagall said, "Harry Potter, it's clear that you're just guessing."
Harry said, firmly, "If you knew it was right or wrong, you wouldn't need guesses. A reliable guess, drawn from my best knowledge, is a basis for a sound decision." Harry bowed in his seat, "Of course, I'd be remiss to fail to ask the expert in her chosen field."
Professor McGonagall smiled softly, and said, "Your reasoning, Mister Potter?"
Harry explained, "The heavenly bodies are powerful entities in the realm of Potion making, but wellwater comes from the Earth. Mother Earth is a steady gravitation, not changeable. Her influence does not wax and wane as the heavens do. She makes a constant, and thus the wellwater does not decrease in potency if you contain it away from her."
Professor McGonagall said, "Well reasoned indeed!" She smiled sadly at him and said, "I am sorry that you have not done more reading towards your Apprenticeship. Finish what you've been given by next week, or there will be more dire consequences than a simple scolding."
In the end, Professor McGonagall didn't even need to scold him. Harry was sad enough from the look on her face.
Somehow, he'd have to get it all done.
*To be clear, Sprout is a gardener. There's dirt in her office. She could clean, but it would just get dirty again. This is not a slur against her.
[a/n: With as many empty rooms as Hogwarts has, Snape has several offices - one that's suitable for meeting with Lucius Malfoy, for example. He has another that he's claimed with the auspices of Potions Master. He has no need for an office for being a spy, of course - that's the point of all the other ones. They function as masks.