The Bath Item Gift Hypothesis
The Big Bang Theory Season 2 Episode 11 (12/15/08)
Leonard is initially jealous of his new co-worker, Dave (Michael Trucco), who's smarter and cooler than him. That is until he gets to work with him and quickly begins to worship the motorcycle-riding, "cool" physicist. After Leonard has an accident while trying to ride Dave's motorcycle, he carries him home where Dave runs into Penny who quickly develops on a crush on him.
Penny pretends to be nerdy as she's flirtiing with Dave and Leonard seems to let it slide. Penny goes out a couple times with Dave but finds out he's married and Leonard is right there to comfort her, though his jealous does come out and she realizes he still has feelings for her.
Sheldon, meanwhile, gets a Christmas gift from Penny and decides he is now forced to get Penny a gift of equal or greater value due to the proper gift giving etiquette. He drags Raj and Wolowitz with him to the mall on a ridiculous quest to get her a gift.