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The Sheldon Character Detail That Only Long-Time Big Bang Theory Fans Know

On CBS' hit series "The Big Bang Theory," nearly all of the characters have their own unique quirks. Raj Koothrappali (Kunal Nayyar) is petrified to speak to women, while Howard Wolowitz (Simon Helberg) sports mysterious alien pins on a daily basis. However, the friend with the most peculiar habits is Sheldon Cooper (Jim Parsons).

Early on in "The Big Bang Theory," a number of interesting personality traits are shared about Sheldon. Before entering someone else's living space, he must always knock three times and loudly state the person's name. This cycle is repeated three times, or until he's invited in. Another unique detail about Sheldon is his overprotectiveness regarding his spot on the far-left side of the couch. It's perfectly situated near the radiator and window, and it has the ultimate view of the television. He also knows when his spot has been messed with. When Penny accidentally shoots his cushion with a paintball and flips it over to hide the bright green stain, Sheldon senses that something is amiss.

Throughout the 12 seasons of "The Big Bang Theory," Sheldon's rhythmic knocking and couch dominance consistently occur. However, there's one aspect of Sheldon that majorly changes over the course of three seasons.

Sheldon was allergic to cats in Season 1 of The Big Bang Theory

In Season 1, Episode 3 ("The Fuzzy Boots Corollary"), it's revealed that Sheldon is allergic to cats, when Leonard Hofstadter (Johnny Galecki) brings one home after being rejected by Penny. Sheldon is opposed to the idea and fears the feline will affect his asthma.

However, in Season 4, Episode 3 ("The Zazzy Substitution"), Sheldon acquires a "clowder" of cats in an attempt to replace his ex-girlfriend Amy Farrah Fowler (Mayim Bialik). Leonard is stunned to see Sheldon acting so affectionately toward the cats, which he endearingly gives names, such as Dr. Robert Oppenheimer and Zazzles. He starts off with five cats, but by the time Leonard brings Sheldon's mother Mary (Laurie Metcalf) over to help, his new pets are scattered across his bedroom, including in the dresser drawers.

In this episode, Sheldon's Season 1 cat allergy is non-existent. This has led many fans to wonder if "The Big Bang Theory" creators forgot about this detail, or if this drastic shift was intentional. On Reddit, u/supbear said, "Allergies can come and go. I was horribly allergic to cats for about four years."

Given Sheldon's tendency to occasionally hide the truth, such as when he lied to Amy about being able drive so that he'd get chauffeured, it's possible Sheldon selfishly faked his allergy in Season 1 to prevent Leonard from getting a cat. It's also suspicious that Sheldon's go-to song to feel better when sick is "Soft Kitty," which is quite literally all about a "little ball of fur."