- An initial explanation put forward on the basis of limited, but factually supported, evidence which has not yet been conclusively proved.
Season 1, Episode 2 - The Big Bran Hypothesis
Aired October 1, 2007
Leonard hopes to make a good impression with Penny by volunteering to sign for a package of hers. When Leonard and Sheldon move the package to her apartment, Sheldon is disturbed by her apparent lack of tidiness. Sheldon's inappropriate mission to tidy Penny's apartment lands him and Leonard in hot water with her.
Season 2, Episode 11 - The Bath Item Gift Hypothesis
Aired December 15, 2008
Leonard starts working with a cool, handsome colleague at work, but things go awry when the colleague starts dating Penny. Meanwhile, Sheldon's obsessive gift-giving etiquette irritates the whole gang, while Penny's Christmas gift for Sheldon leads to a memorable moment.
Season 4, Episode 10 - The Alien Parasite Hypothesis
Aired December 9, 2010
Amy is confused by an unusual sensation she felt in the presence of Penny's ex, Zack. With Sheldon's help, Amy realizes she was experiencing feelings of sexual arousal, so Sheldon sets out to help satisfy her needs. Meanwhile, Howard and Raj battle to find out who is the superhero and who is the sidekick in their friendship.
Season 5, Episode 2 - The Infestation Hypothesis
Aired September 22, 2011
When Penny takes in furniture she found in the street, germophobe Sheldon is horrified and urges Penny to remove the chair from the building. Amy finds herself caught in the middle of the argument between Sheldon and Penny. Meanwhile, Leonard is still adjusting to his long-distance relationship with Priya and tries to spice things up between them.
Season 5, Episode 13 - The Recombination Hypothesis
Aired January 19, 2012
Leonard and Penny might start things up again when Leonard offers Penny a spur of the moment invitation to dinner, on the 100th episode of The Big Bang Theory.