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Professor McGonagall seemed highly amused. "Would you like a futher demonstration, Mr. Potter?"

"You don't have to," Harry said. "We've performed a definitive experiment. But..." Harry hesitated. He couldn't help himself. Actually, under the circumstances, he shouldn't be helping himself. It was right and proper to be curious. "What else can you do?"

McGonagall turned into a cat.

Harry scrambled back unthinkingly, backpedaling so fast that he tripped over a stray stack of books and landed hard on his bottom with a thwack. His hands came down to catch himself without quite reaching properly, and there was a warning twinge in his shoulder as the weight came down unbraced.

At once the small tabby cat morphed back up into a robed woman. "I'm sorry, Mr. Potter," McGonagall said, sounding sincere, though her lips were twitching toward a smile. "I should have warned you."

Harry was breathing in short pants. His voice came out choked. "You can't DO that!"

"It's only a Transfiguration," said McGonagall. "An Animagus transformation, to be exact."

"You turned into a cat! A SMALL cat! You violated Conservation of Energy! That's not just an arbitrary rule, it's implied by the form of the quantum Hamiltonian! Rejecting it destroys unitarity and then you get FTL signaling! And cats are COMPLICATED! A human mind can't just visualize a whole cat's anatomy and, and all the cat biochemistry, and what about the neurology? How can you go on thinking using a cat-sized brain?"

McGonagall's lips were twitching harder now. "Magic."

"Magic isn't enough to do that! You'd have to be a god!"

McGonagall blinked. "That's the first time I've ever been called that."

Harry Potter and the Methods of Rationality, Chapter 2