The Gallon or the Gallows
"Let's see. We need 3 toad eyes, 27 hairs from an elephant, 1 ounce of dragon blood, a pinch of paprika, 1 tiger tooth, 13 four-leaf clovers, and exactly 1 gallon of water. Hurry, please, or you will regret it!"
The head witch's words ring in your ears as she sends you from the school to gather the ingredients for the invisibility potion. You can't mess this up! The witch has already threatened to expel you after your levitating spell went awry - it took forever to get all of the classroom's furniture off of the ceiling.