When Penny locks herself out of the apartment, she comes to hang out with Sheldon in his as he plays Age of Conan. Penny, frustrated by her recent failures in life soon plays and becomes addicted to Conan.
Sheldon: I'm a physicist. I have a working knowledge of the entire universe and everything it contains Penny: Who's Radiohead? Sheldon [after twitching for a minute]: I have a working knowledge of the important things
It's very simple. Scissors cuts paper, paper covers rock, rock crushes lizard, lizard poisons Spock, Spock smashes scissors, scissors decapitates lizard, lizard eats paper, paper disproves Spock, Spock vaporizes rock, and, as it always has, rock crushes scissors