Many people in The Lightning Thief, both mortal and immortal, hope to deceive our narrator and others for the sake of causing chaos and violence. Lying doesn't necessarily involve saying something untrue in this novel. Often lying simply involves keeping information from someone (like the way Percy chooses not to tell Chiron everything that the Oracle told him). Percy's entire quest is built upon a deception, and so we could say that the entire novel is built upon deception. It gets tricky, though, when certain characters know how to read minds or eavesdrop on dreams. It becomes clear that lying is not a very easy task when gods and goddesses are involved.