Polyjuice Potion

From Hpwiki

The Polyjuice Potion allows the drinker to change their appearance and voice into those of another person, though they will turn back after one hour. The ingredients are as follows: lacewing flies, leeches, fluxweed (picked at the full moon), knotgrass, powdered horn of a Bicorn, shredded skin of a Boomslang, and a piece of whoever you wish to change into (who should always be a person; the transformation to animals with this potion has undesired effects).

In second year Hermione Granger manages to brew this complicated potion with the goal of spying on Draco Malfoy. Harry and Ron turn into Crabbe and Goyle, while Hermione is turned into a half-cat. The potion has also been used by Barty Crouch, Jr. to impersonate Alastor Moody in Harry's fourth year. It also happens to be part of Harry's theoretical Potions OWL exam.