Snape closed the door to one of the bathrooms on the ground floor, holding the goblet with the acid-red Everlasting Polyjuice Potion in it. To be honest, he was quite nervous, but excited too. He placed Sirius's steely gray robes on the sink, leaving his own black robes on the floor, and prepared to take the potion. This was it…

Essence of Sirius. Snape winced and poured it down his throat. It tasted like strawberries.

As soon as the goblet had been swallowed, Snape felt the effects that of course he knew were coming, even though he had never used this potion before. Since he knew what the effects were going to be, he was better prepared, but he was afraid to look in the mirror…

Snape's hands were soon thin and clawlike; in fact, all of him felt suddenly very, very thin. He felt a strange jumping sensation in his stomach as he noticed his Dark Mark fade away into nothing, although strange scabs did appear on his right hand, almost as if something had attacked that hand. He gasped as he shot up in height; his shoulders broadened; in fact, he felt like his body was becoming more well-built and sturdy, less reedy, but again so thin, almost like a skeleton. He could see his own ribs. Well, Sirius's ribs, that is. His head felt extremely light—he wondered why, until he realized it was because there was no grease in his hair. His chin felt a little itchy as he obtained Sirius's stubble, proof of the fact that he couldn't be bothered to pick up a razor more than three or four times a week. Finally, he picked the gray robes off the sink and slipped them over his head. When he put on the loose robes, his thinness wasn't so conspicuous.

There he was, in exact likeness: Severus Snape, disguised as Sirius Black, and he would be that way until he took the antidote.

Snape crept out of the bathroom. Walking in a new body felt weird; he still had his prowling walk, but it looked strange with this different build. It wasn't long before he located Sirius in the foyer, wearing a midnight-blue wool dressing gown.

"Woah," said Sirius. "Freaky."

Snape just rolled his eyes and shoved his robes into Sirius's hands. "Hurry, take the potion before anyone notices us."

…

When Sirius got to his room he threw his dressing gown onto the bed and downed Essence of Snape in one gulp. It tasted exactly like dark chocolate. Sirius had never liked dark chocolate, but that was what it tasted like.

The effects of the potion were not pleasant. Sirius wondered for a moment if he was going to throw up, but then he felt a hot, burning sensation from his stomach to his feet—but that wasn't the weird part. Sirius shrunk slightly in height; he became less thin and he was appalled to see the Dark Mark form on his right arm. His chin turned from rough and stubbly to smooth; he still felt like he was shrinking slightly (except in the nose department), taking on Snape's batlike form. His hair felt disgusting—he would need to do damage control right away as soon as he got to Snape's house. It felt like he was wearing a hat made of grease. When everything was finished, he put the black robes on, and stared at himself in his bedroom mirror. Snape stared back at him.

Freaky, indeed.

Sirius sighed and Apparated downstairs, where the real Snape was waiting for him. Snape-as-Sirius didn't look shocked, but he did raise one eyebrow.

"Stop crouching," said Sirius. "You've got to stand up straight if you want to look like me."

Snape-as-Sirius straightened his back, and Sirius-as-Snape hunched his slightly.

"Better?" said Snape-as-Sirius dryly.

"Sure, but you may need some time to perfect this," said Sirius-as-Snape, who then strutted across the room with his hip-swaggering gait, which Snape had seen him do a lot in school, except this time he was seeing himself doing it, and…well…Snape had always wanted to be able to do that walk, and now here he was, doing it. Sort of.

"You need to smile less," Snape snapped.

Sirius grinned obnoxiously; Snape scowled.

"Here's my address," said Snape, conjuring a piece of paper and writing supplies. When his address was written down, he handed it to Sirius. "Don't forget to study Occlumency and act like me. Here, now go Apparate there."

"Thanks, Sirius," said Sirius gaily, "but I think I'll walk."