Polyjuice Potion

"Immediately, his insides started writhing as though he'd just swallowed live snakes -- doubled up, he wondered whether he was going to be sick -- then a burning sensation spread rapidly form his stomach to the very ends of his fingers and toes -- next, bringing him gasping to all fours, came to a horrible melting feeling, as the skin all over his body bubbled like hot wax -- and before their eyes, his hands began to grow, the fingers thickened, the knuckles were bulging like bolts -- his shoulders stretched painfully and a prickling on his forehead told him that his hair was creeping down toward his eyebrows -- his robes ripped as his chest expanded like a barrel bursting its hoops -- his feet were agony in shoes four sizes too small --"

The trio used Polyjuice that they stole from Moody to infiltrate the Ministry of Magic to search for Salazar Slytherin's Locket in September of 1997. Harry, Ron and Hermione disguised themselves as Albert Runcorn, Reginald Cattermole, and Mafalda Hopkirk, respectively.

The following year, Hermione used the remaining Polyjuice Potion again as part of the trio's plan to break in to Gringotts in order to retrieve Helga Hufflepuff's Cup from theLestrange Vault. Using a strand of Bellatrix Lestrange's hair that had fallen on her sweater while Bellatrix interrogated her at Malfoy Manor, Hermione assumed her form.^[3] However, there wasn't enough, so Ron had to simply use cosmetic design to disguise himself. The effects of the potion were washed away by the Thief's Downfall at the bank.