Truth Serum is a potion that makes others truthful and highly suggestive.
Quentin Coldwater slipped the serum in Henry Fogg's tea before asking to speak with the Dean in his office, where Coldwater told Fogg he wanted to know what the truth behind Eliza and The Beast was. When Fogg refused to answer, Coldwater asked him how he lost his virginity, and Fogg went on to tell him about his experience with an older woman while in his twenties before he realized what Coldwater had done. When Coldwater asked who Eliza really was, Fogg revealed that her real name was Jane Chatwin, much to Coldwater's shock. Fogg then explained that the gift Jane was given in The Girl Who Told Time by Ember was what allowed her to alter time on a cosmic scale, and that Jane created 39 previous alternate timelines in her pursuit to stop The Beast.