Bubble Juice is supposedly a magical juice. It apparently bears a resemblance to Veritaserum, as Harry Potter once thought the truth serum that Professor Snape stored in his storeroom to be bubble juice. Snape accused Harry of stealing Gillyweed and the ingredients for Polyjuice Potion, and thus threatened to use Veritaserum on him. It turned out that Barty Crouch Jr (disguised as Alastor Moody) was the real culprit.