Veritaserum

Veritaserum is a very potent truth potion. It was first mentioned in the book Harry Potter and the Goblet of Fire.

Veritaserum is clear, colorless, and odorless, and has pretty much the same appearance as water. When consumed, it forces the drinker to tell the truth, to the point that he finds it impossible to lie. It can be mixed with any drink.

It was used on Barty Crouch, Jr. towards the end of Goblet, when Albus Dumbledore discovered that Crouch was disguised as the Defence Against the Dark Arts teacher, "Mad-Eye" Moody. Severus Snape also threatened Harry Potter with it in the same book.