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.
Penny, Bernadette and Amy are seen here during a night out. It's a scene from the episode "The Alien Parasite Hypothesis."

The Big Bang Theory Season 4 Episode 10 Quotes

Wolowitz: He got bit by a radioactive rat.
Raj: Did he get superpowers?
Wolotiz: No, he got five stitches and a tetanus shot.

Penny: He wasn't intellectually stimulating enough.
Bernadette: Couldn't you just fool around with him and then listen to NPR?
Penny: Wouldn't help. Zack couldn't even spell NPR.