Recently, we've been teaching about the truth and lying during family home evening. The oldest three indicated they understood, and my three-year-old son seemed to grasp the general concept -- lies are bad -- so we were happy with our progress.
Then yesterday morning after my son got out of the shower, I wrapped a towel around him to dry off and stay warm. The towel kept slipping off his shoulders, and he was getting seriously frustrated. Finally, he exploded with the foulest language he could think of: "Towels are a LIE!"