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.
Raj: Well, to paraphrase Shakespeare: It's better to have loved and lost than to stay home every night and download increasingly shameful pornography. Penny: Oh... you poor baby. Raj: What's wrong with me, Penny? Penny: Nothing, nothing. You know, if we weren't friends - and you hadn't brought up that creepy pornography story - I'd be on you like the speed of light squared on matter to make energy. Raj: Hey, you totally got that right. E = MC squared. Penny: I listen. I have no idea what it means, but I listen.