The Spaghetti Catalyst

​"The Spaghetti Catalyst" is the 20th episode of The Big Bang Theory's third season. It aired on May 3rd 2010.

Summary
Leonard and Penny are still fresh from their break-up, and it is time for the guys to "pick sides" about who will remain friends. But Sheldon, as usual, is oblivious to this social convention and keeps contact with Penny, going as far as inviting himself over for spaghetti (with cut-up hot dogs reminiscent of his mother). When Sheldon reveals to the guys how he's been talking to Penny, Howard explains to him how he cannot maintain friendship both with her and with Leonard.

In order not to arise suspicions, Sheldon eats at his apartment, then sneaks off to the refrigerator to grab a pack of hot dogs to take over to Penny's. Just as he's about to leave with the excuse of taking a walk, however, Howard leaves as well, insisting that the two go down together. Sheldon tries to shake off Howard, but ends up accompanying him right to his parked scooter. When he finally gets hid of his friend, Sheldon bumps into a dog which smells the hot dogs, and has to run for his life. He manages to shake the dog off and arrives at Penny's in a state of disgrace and holding a single hot dog. The two eat spaghetti together, and Sheldon ends up with stomachache from eating too much.

Sheldon has trouble keeping his friendship with Penny secret from his roommate, and ends up recounting his continued friendship to Leonard in the middle of the night. Leonard, however, seems not to care about this. Some time later, Penny and Leonard bump into each other in the laundry room, where the two try to keep things friendly between each other. Leonard explains how he's fine with Sheldon and Penny being friends, but seems upset when he learns that Penny is taking Sheldon for sheets, shoes and to a trip to Disneyland. Leonard gives Penny suggestions on how to take care of Sheldon at Disneyland, as he is familiar with the events of Sheldon eating junk food and throwing it up after he goes on the Space Mountain.

The remainder of this episode showcases Leonard and Penny as "separated parents" to their "son" Sheldon. Penny, it seems, has ignored Leonard's tips and Sheldon reacts exactly as Leonard had predicted. The two see Sheldon off to bed, and Penny admits to Leonard how they can remain friends despite their break-up. Leonard off-handedly tries to suggest they remain friends with benefits.

Title reference
Sheldon's spaghetti dinner with Penny is the catalyst for repairing Leonard and Penny's friendship.

Quotes
Sheldon: Go Team Leonard! (Holds a "L" on his forehead which typcially stands for "loser")

Trivia
*The t-shirt with the starburst pattern that Sheldon wears is the Star Sapphire shirt from the Green Lantern series. The Star Sapphires preside over the emotion of love, reflecting Sheldon's split affections for Penny and Leonard.
 * Sheldon is afraid of the Disney Dog Goofy, but is alright with Pluto.
 * Sheldon does't complain about the tangerine chicken in this episode, unlike in The Tangerine Factor.