Veritaserum

A powerful truth serum, three drops were enough to cause any drinker to reveal their very innermost secrets. Under strict control by the Ministry, Veritaserum was odourless, colourless and tasteless and could be sneaked into the drinker's water without them knowing. Some wizards and witches were able to resist its effects, however, and an antidote was possible to prevent the potion taking full effect. Snape threatened Harry with the potion when he suspected he had been stealing.