The Gorilla Experiment

Summary
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 watch him conduct an experiment at work. Wolowitz is jealous of this and accuses Leonard of being interested in Bernadette. The following day, Howard confronts Leonard. Bernadette arrives to watch Leonard's experiment, at which point Howard apologizes, and they kiss and make up.

Meanwhile, Penny seeks out Sheldon to teach her about physics so that she might be able to relate to Leonard and discuss his work. Later at 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.

Title reference: Penny's suggestion that Sheldon should treat teaching her physics as an experiment. Sheldon subsequently names the experiment "The Gorilla Project," after Koko the gorilla who learned sign language.