- Something felt or done in direct response to another action.
- A chemical process whereby two or more substances interact and change each other as a result of the contact.
Season 1, Episode 16 - The Peanut Reaction
Aired May 12, 2008
When Penny finds out that Leonard has never celebrated his birthday, she sets out to throw him a surprise party. However, a quick trip to an electronics store with Sheldon for a birthday present takes longer than she expected, forcing Howard to take drastic action to stall Leonard.
Season 3, Episode 13 - The Bozeman Reaction
Aired January 18, 2010
After their apartment is robbed, Leonard and Sheldon get Howard and Raj to install a complex security system. When Sheldon still doesn't feel safe there, he decides to leave Pasadena and begins searching for his next home.
Season 4, Episode 23 - The Engagement Reaction
Aired May 12, 2011
After Howard finally tells his mother he's engaged to Bernadette, she ends up in the hospital. Meanwhile, Leonard is concerned when Penny and Priya bond.
Season 5, Episode 5 - The Russian Rocket Reaction
Aired October 13, 2011
When Sheldon's nemesis Wil Wheaton invites him and Leonard to a party at his house, Sheldon is unhappy that his friend is going to the party. Meanwhile, Howard and Bernadette argue when Howard is offered an incredible opportunity to go to scape.
Season 6, Episode 24 - The Bon Voyage Reaction
Aired May 16, 2013
When Leonard receives an exciting job proposal, Penny worries about their relationship, while Sheldon deals with feelings of jealousy. Meanwhile, when Raj wants more from his relationship with Lucy, he pushes too far.
Season 9, Episode 16 - The Positive Negative Reaction
Aired February 18, 2016
When the gang finds out about Bernadette's pregnancy, they celebrate by singing karaoke. Also, Bernadette is concerned when Wolowitz freaks out over her pregnancy announcement, and Wolowitz is convinced he must make more money to provide for the baby.
Season 11, Episode 2 - The Retraction Reaction
Aired October 2, 2017
Leonard fears for his job when he gives a downbeat assessment of the state of physics research in a national radio interview. Meanwhile, Amy and Bernadette realize they both feel they have to hide success in their careers from their significant others.