Season 4, Episode 3 - The Zazzy Substitution
Aired October 7, 2010
When Sheldon and Amy argue over the importance of their respective fields, they decide to terminate their relationship. As Sheldon struggles to handle the break-up, he turns to feline companionship. Leonard gets concerned about his friend and calls in help.
Season 9, Episode 23 - The Line Substitution Solution
Aired May 5, 2016
Sheldon hires Stuart to spend the day with Amy when he'd rather go to a movie screening. Also, Leonard's mother, Beverly, comes to town and Penny struggles to make a connection with her.