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Ender does not trust anyone, and one of the manifestations of that distrust is the suggestion that everyone lies, especially the adults (and in particular teachers) towards the children. For instance, Graff lies to Ender multiple times, the most obvious time being when he and Mazer pretend that the battles Ender is fighting against the buggers are actually just Mazer's battle scenarios. Ender ended up destroying almost all the buggers while thinking that he was just winning games. Even the young Valentine is insincere insofar as she writes under the pseudonym Demosthenes
Sometimes, lies are more dependable than the truth
This idea is submitted by Ender very early in the novel, when the nurse tells him that taking his monitor out will not be painful. Ender points out that since adults only say such a thing when it is going to hurt, children can always depend on the lie to warn them of the pain. In this way, the idea that everybody lies almost becomes a good thing, since it means that they are easier to figure out. A lie reveals a truth about a person's motivations.