"Transfiguration is some of the most complex and dangerous magic you will learn at Hogwarts," said Professor McGonagall. There was no trace of any levity upon the face of the stern old witch. "Anyone messing around in my class will leave and not come back. You have been warned."
Her wand came down and tapped her desk, which smoothly reshaped itself into a pig. A couple of Muggleborn students gave out small yelps. The pig looked around and snorted, seeming confused, and then became a desk again.
The Transfiguration Professor looked around the classroom, and then her eyes settled on one student.
"Mr. Potter," said Professor McGonagall. "You only received your schoolbooks a few days ago. Have you started reading your Transfiguration textbook?"
"No, sorry professor," Harry said.
"You needn't apologise, Mr. Potter, if you were required to read ahead you would have been told to do so." McGonagall's fingers rapped the desk in front of her. "Mr. Potter, would you care to guess whether this is a desk which I Transfigured into a pig, or if it began as a pig and I briefly removed the Transfiguration? If you had read the first chapter of your textbook, you would know."
Harry's eyebrows furrowed slightly. "I'd guess it'd be easier to start with a pig, since if it started as a desk, it might not know how to stand up."
Professor McGonagall shook her head. "No fault to you, Mr. Potter, but the correct answer is that in Transfiguration you do not care to guess. Wrong answers will be marked with extreme severity, questions left blank will be marked with great leniency. You must learn to know what you do not know. If I ask you any question, no matter how obvious or elementary, and you answer 'I'm not sure', I will not hold it against you and anyone who laughs will lose House points. Can you tell me why this rule exists, Mr. Potter?"
Because a single error in Transfiguration can be incredibly dangerous. "No."
"Correct. Transfiguration is more dangerous than Apparition, which is not taught until your sixth year. Unfortunately, Transfiguration must be learned and practised at a young age to maximise your adult ability. So this is a dangerous subject, and you should be quite scared of making any mistakes, because none of my students have ever been permanently injured and I will be extremely put out if you are the first class to spoil my record."
Several students gulped.
Professor McGonagall stood up and moved over to the wall behind her desk, which held a white wooden board. "There are many reasons why Transfiguration is dangerous, but one reason stands above all the rest." She produced a marker seemingly from thin air, and sketched letters in bright red; which she then underlined, using the same marker, in blue:
TRANSFIGURATION IS NOT PERMANENT!
"Transfiguration is not permanent!" said Professor McGonagall. "Transfiguration is not permanent! Transfiguration is not permanent! Mr. Potter, suppose a student Transfigured a block of wood into a cup of water, and you drank it. What do you imagine might happen to you when the Transfiguration wore off?" There was a pause. "Excuse me, I should not have asked that of you, Mr. Potter, I forgot that you are blessed with an unusually pessimistic imagination -"
"I'm fine," Harry said, swallowing hard. "So the first answer is that I don't know," the Professor nodded approvingly, "but I imagine there might be... wood in my stomach, and in my bloodstream, and if any of that water had gotten absorbed into my body's tissues - would it be wood pulp or solid wood or..." Harry's grasp of magic failed him. He couldn't understand how wood mapped into water in the first place, so he couldn't understand what would happen after the water molecules were scrambled by ordinary thermal motions and the magic wore off and the mapping reversed.
McGonagall's face was stiff. "As Mr. Potter has correctly reasoned, he would become extremely sick and require immediate Flooing to St. Mungo's Hospital if he was to have any chance of survival. Please turn your textbooks to page 5."
Even without any sound in the moving picture, you could tell that the woman with horribly discolored skin was screaming.
"The criminal who originally Transfigured gold into wine and gave it to this woman to drink, 'in payment of the debt' as he put it, received a sentence of ten years in Azkaban. Please turn to page 6. That is a Dementor. They are the guardians of Azkaban. They suck away at your magic, your life, and any happy thoughts you try to have. The picture on page 7 is of the criminal ten years later, on his release. You will note that he is dead - yes, Mr. Potter?"
"Professor," Harry said, "if the worst happens in a case like that, is there any way of maintaining the Transfiguration?"
"No," Professor McGonagall said flatly. "Sustaining a Transfiguration is a constant drain on your magic which scales with the size of the target form. And you would need to recontact the target every few hours, which is, in a case like this, impossible. Disasters like this are unrecoverable!"
Professor McGonagall leaned forwards, her face very hard. "You will absolutely never under any circumstances Transfigure anything into a liquid or a gas. No water, no air. Nothing like water, nothing like air. Even if it is not meant to drink. Liquid evaporates, little bits and pieces of it get into the air. You will not Transfigure anything that is to be burned. It will make smoke and someone could breathe that smoke! You will never Transfigure anything that could conceivably go inside anyone's body by any means. No food. Nothing that looks like food. Not even as a funny little prank where you mean to tell them about your mud pie before they actually eat it. You will never do it. Period. Inside this classroom or out of it or anywhere. Is that well understood by every single student?"
"Yes," said Harry, Hermione, and a few others. The rest seemed to be speechless.
"Is that well understood by every single student?"
"Yes," they said or muttered or whispered.
"If you break any of these rules you will not further study Transfiguration during your stay at Hogwarts. Repeat along with me. I will never Transfigure anything into a liquid or gas."
"I will never Transfigure anything into a liquid or gas," said the students in ragged chorus.
"Again! Louder! I will never Transfigure anything into a liquid or gas."
"I will never Transfigure anything into a liquid or gas."
"I will never Transfigure anything that looks like food or anything else that goes inside a human body."
"I will never Transfigure anything that is to be burned because it could make smoke."
"You will never Transfigure anything that looks like money, including Muggle money," said Professor McGonagall. "The goblins have ways of finding out who did it. As a matter of recognised law, the goblin nation is in a permanent state of war with all magical counterfeiters. They will not send Aurors. They will send an army."
"I will never Transfigure anything that looks like money," repeated the students.
"And above all," said Professor McGonagall, "you will not Transfigure any living subject, especially yourselves. It will make you very sick and possibly dead, depending on how you Transfigure yourself and how long you maintain the change." Professor McGonagall paused. "Mr. Potter is currently holding up his hand because he has seen an Animagus transformation - specifically, a human transforming into a cat and back again. But an Animagus transformation is not free Transfiguration."