Sheldon: My mother was at bible study. I walked in the house expecting to find it empty, and I heard a sound coming from my parents' bedroom. When I opened the door, I saw my father having relations with another woman.
Penny: Oh, that's awful!
Sheldon: I know. It's also why I never open a door without knocking three times. I mean, the first one's traditional, but two and three are for people to get their pants on.