Season 2: Episode 20 - The Hofstadter Isotope

The Hofstadter Isotope: In this episode Penny is hit on by the sales clerk in a comic book store that the guys frequent. Leonard is devastated at this turn of events. He can't believe that Penny would date one of "them" if it isn't him. In the mean time, Sheldon gets in the middle of the budding romance by constantly challenging the new guy to engage in a Batman dialogue. The new guy is such a nerd, that he falls right into Sheldon's hands. Penny ends up asleep on the couch while the guys hash it out over what Robin's real name is.