Harry Potter and the Methods of Rationality

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."

Professor McGonagall took a small piece of wood out of her pocket. With a tap of her wand it became a glass ball. Then she said "Crystferrium!" and the glass ball became a steel ball. She tapped it with her wand one last time and the steel ball became a piece of wood once more. "Crystferrium transforms a subject of solid glass into a similarly shaped target of solid steel. It cannot do the reverse, nor can it transform a desk into a pig. The most general form of Transfiguration - free Transfiguration, which you will be learning here - is capable of transforming any subject into any target, at least so far as physical form is concerned. For this reason, free Transfiguration must be done wordlessly. Using Charms would require different words for every different transformation between subject and target."

Professor McGonagall gave her students a sharp look. "Some teachers begin with Transfiguration Charms and move on to free Transfiguration afterwards. Yes, that would be much easier in the beginning. But it can set you in a poor mold which impairs your abilities later. Here you will learn free Transfiguration from the very start, which requires that you cast the spell wordlessly, by holding the subject form, the target form, and the transformation within your own mind."

"And to answer Mr. Potter's question," Professor McGonagall went on, "it is free Transfiguration which you must never do to any living subject. There are Charms and potions which can safely, reversibly transform living subjects in limited ways. An Animagus with a missing limb will still be missing that limb after transforming, for example. Free Transfiguration is not safe. Your body will change while it is Transfigured - breathing, for example, results in a constant loss of the body's stuff to the surrounding air. When the Transfiguration wears off and your body tries to revert to its original form, it will not quite be able to do so. If you press your wand to your body and imagine yourself with golden hair, afterwards your hair will fall out. If you visualise yourself as someone with clearer skin, you will be taking a long stay at St. Mungo's. And if you Transfigure yourself into an adult bodily form, then, when the Transfiguration wears off, you will die."