The Gorilla Experiment (2009)

Bernadette, in the guys' apartment for dinner, shows an interest in physics, particularly in the work that Leonard is conducting. Leonard offers her the chance to come tour his lab and watch him conduct an experiment. Penny gets jealous because she doesn't have enough background in physics to talk to Leonard about his job. She therefore seeks out Sheldon to teach her physics so that she would be able to relate to Leonard and discuss his work. In spite of his initial refusal, he agrees to take on the challenge of teaching physics to Penny. At the end of the show, while having dinner in the apartment, Penny spouts off a clearly prepared spiel which Sheldon has coached her in, repeating Sheldon's viewpoint that Leonard's experiment is functionally identical to the one conducted by a Dutch team of scientists.

Bernadette's interest in Leonard's work makes Howard jealous and accuses Leonard of being interested in Bernadette. Leonard then texts Bernadette that because Wolowitz is jealous, it may be a good idea to cancel the tour. Bernadette receives the text message right before she was going to have sex with Howard, becomes enraged and subsequently storms out. The following day, Howard confronts Leonard at his lab. Bernadette then arrives to Leonard's lab to watch Leonard conduct an experiment, at which point Howard apologizes, and they kiss and make up.

Recurring characters: Melissa Rauch as Bernadette Rostenkowski

Title reference: Sheldon's view that teaching physics to Penny is equivalent in challenge to teaching sign language to Koko the gorilla.