Novel

Uglies by Scott Westerfeld

...At the novel, the main character, Tally, pretends to defend one size of a cause but she is actually a spy who is trying to get her new "friends" down. But as she spends so much time with the people she was trying to let down, she ends up wanting to defend them for true. However, she can't change sides anymore, because they would never believe in someone that lied to them once.
I learned from the novel Uglies, that if you hide something from someone for a long time, it's very hard to make people believe in you, you won't be trusted in big matters. Therefore, you should never tell lies in first place...