Lily Aldrin was one of the more opinionated characters in How I Met Your Mother. If she didn't agree with something or didn't like someone's behavior, she had no qualms about speaking her mind. In some cases, the character played by Alyson Hannigan would use her own form of punishment known as "Aldrin Justice" to get back at the people who wronged her.
Aldrin Justice was Lily's personal form of punishment that derived from her time as a kindergarten teacher. Whenever her students misbehaved, she would take away one of their toys as punishment. Lily adapted this technique to her adult life by taking items of value from people who offended her, hurt her, or treated her friends unfairly.
Aldrin Justice was used a number of times throughout How I Met Your Mother. Lily's friends weren't too keen on the idea of her stealing from people, especially Marshall. The gang always fought to convince Lily to return the stolen items, and sometimes she did — but not always. The use of Aldrin Justice usually held further consequences and often taught Lily, or her friends, other important life lessons.
Every Episode Lily Used Aldrin Justice In How I Met Your Mother
"Aldrin Justice" (Season 2 Episode 6): The first time Lily used Aldrin Justice outside of her classroom was when she stole a pair of khakis from a store when the salesperson was rude to Robin. The pants were then given to Ted as a gift and never returned. This story was explained after Lily stole the prized baseball owned by Ted's condescending boss, Hammond Druthers (Bryan Cranston). The baseball was signed three times by Pete Rose and served as a special possession to Hammond. Lily refused to give it back so Ted had no choice but to fire her from her job as an office assistant; however, her form of justice subsequently inspired him to stand up to his boss.
"How Lily Stole Christmas" (Season 2 Episode 11): Although Lily didn't refer to her actions in this episode as Aldrin Justice, she did steel something as a way to get back at someone. She turned the apartment into a Winter Wonderland for Marshall and Ted to enjoy during the holidays. Upon learning that Ted called Lily a "grinch" (which was code for a swear word) when she left Marshall, Lily stole the Christmas decorations and brought them to her own apartment. She then threatened to call Ted's parents about his use of foul language, which is another punishment she does with her students.
"Blitzgiving" (Season 6 Episode 10): Before Ted entered in a relationship with Zoey Pierson (Jennifer Morrison), the two were enemies. The night before Ted's big planned Thanksgiving get-together, his friends encountered Zoey at a bar and became friends with her. When Ted's dinner for his friends fell through, they found themselves invited to Zoey's place for dinner. Ted and Zoey continued to argue resulting in the gang getting kicked out. To punish Zoey for kicking them out, Lily stole a toy turkey. When the group later learned the toy was for Zoey's stepdaughter, Hannah, they go back to make amends.
"The Ashtray" (Season 8 Episode 17): Due to her experience with art, The Captain (Kyle MacLachlan) asked Lily to see a painting he bought at his apartment after at art gallery showing. When she commented that another painting would have looked better than the one he bought, The Captain tried to insult her by referring to her as "just a kindergarten teacher." Lily was offended so she stole an expensive ashtray from his apartment. Marshall then convinced her to return the ashtray and when she did, The Captain didn't even know it was missing.
"Bass Player Wanted" (Season 9 Episode 13): The How I Met Your Mother episode focused on how the Mother met Marshall and revealed she took advice about how to use Aldrin Justice. The Mother, aka Tracy, started a band in college, but the lead vocalist was trying to take over and replace her. To get revenge, Tracy stole his van before their gig at the Farhmapton Inn for Barney and Robin's wedding. After picking up Marshall and Marvin, she returned the van and took back her spot in the band.