Episode: "The First Secret"
Air Date: 10/19/11

Aside from being a great excuse to get the uniformly hot cast of Pretty Little Liars to dress up in revealing costumes, "The First Secret" serves as a flashback episode that depicts the start of a never-ending run of lies and deceit for the girls, before Alison disappeared. So basically, they've always been pretty, and only recently started lying. Got it.