What potion allows you to transform into someone else?

The story behind the answer

Polyjuice Potion is correct because it lets the drinker assume the exact physical form of another person for about an hour—provided you add a bit of that person (like a hair) to the brew. It’s notoriously difficult, requiring a month to prepare with ingredients like lacewing flies stewed 21 days and boomslang skin, and is detailed in the book Moste Potente Potions. Hermione, Harry, and Ron famously use it in Chamber of Secrets, and Death Eaters rely on it later for deception.

Remember: “Poly” means many—think many faces from one “juice.” Don’t confuse it with Transfiguration (a branch of magic, not a potion) or the innate Metamorphmagus ability (like Tonks), which doesn’t need a potion at all.