12 Best The Big Bang Theory Quotes from The Hot Tub Contamination (10x05) - Sheldon Cooper: When I opened the door, I saw my father having relations with another woman. Penny Hofstadter: Oh, that's awful! Sheldon Cooper: 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.