CBS

Season 3: Episode 20 - The Spaghetti Catalyst

The Spaghetti Catalyst: Sheldon tries to hide from Leonard that he is still "seeing" Penny. When Penny runs into Sheldon in the hallway and he expresses to her that he would like to remain friends, even though she and Leonard are no longer "coitusing," Penny is pleasantly surprised at Sheldon's overture, and invites him to dinner. Sheldon then feels compelled to hide his continued friendship with Penny form Leonard. In doing so, he eats dinner and home with the guys, and then pretends he is going for a walk and sneaks over to Penny's for another dinner. Finally the lying breaks him down, and he tells Leonard that he is still "seeing” Penny. Leonard does not mind. Penny and Leonard embark on a divorced-parent-like relationship with Sheldon. At the end of the episode, they agree that they can be friends.