The Alien Parasite Hypothesis
The Big Bang Theory Season 4 Episode 10 (12/09/10)
Amy gets sexually aroused when they run into Penny's ex, Zack. Well actually it takes Sheldon and her studying the symptoms to figure it out. How does Sheldon deal with his "girlfriend" being horny? By trying to set her up with Zack. She goes to talk to him, realizes he's a moron, and gives up. She tries holding Sheldon's hand as an experiment... and it fails.
Meanwhile, Raj and Wolowitz have a competition to see who would be the superhero and who would be the sidekick.