"Here," Hermione cried, "put this in and stir counter-clockwise!"

Harry fumbled with the lid and asked, "What exactly is mint jelly supposed to do, Hermione?" even as he scooped it into the cauldron.

She was already busy dumping another jar into the second cauldron and didn't answer, but Harry got his answer well enough when they tipped the potion over the castle wall and the damn thing exploded over the green, eating through every Death Eater's wand.