A jar of powdered rat spleen

"Usefull for some potions, including Shrinking Solution."
Rat spleen is a common ingredient used in the preparation of various potions. It is a required ingredient in the Shrinking Solution and in Dragon dung Fertiliser.

In 1993, Neville Longbottom added too many rat spleens and too much leech juice into his Shrinking Solution, making it poisonous. Hermione Granger helped Neville fix the potion before Professor Snape had a chance to test it on Neville's toad, Trevor.