He was just telling everyone to look at the perfect way Malfoy had stewed his horned slugs when clouds of acid green smoke and a loud hissing filled the dungeon. Neville Longbottom had somehow managed to melt Seamus's cauldron into a twisted blob, and their potion was seeping across the stone floor, burning holes in people's shoes. Within seconds, the whole class was standing on their stools while Neville, who had been drenched in the potion when the cauldron collapsed, moaned in pain as angry red boils sprang up all over his arms and legs.