Sheldon and Amy do a scientific evaluation when she gets turned on by Zach. They determine that she's horny. He wants her to go out with Zach, but instead she tries a hand-holding experiment with Sheldon.
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