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Sheldon is Leonard's best friend, roommate, and colleague, who appears to have OCD, beginning with suspicion of it in episode 102. He's a physicist like Leonard and also keeps a white board in the living room for scientific theories. Unlike Leonard, he embraces his genius fully and doesn't mind sharing details of his life that Leonard finds embarrassing, such as their main weekend activity being Klingon Boggle tournaments until 1 AM. It's insinuated that Sheldon's genius is concentrated fully on science and that his IQ is higher than Leonard's; he tends to question Leonard's theories and has a larger background full of behavior common to child prodigies.

When he was a child, he conducted experiments involving the height of stairs and deduced that if a step on a staircase is 2 millimeters off or more, a person is always likely to trip (this resulted in his father breaking his clavicle). At age 14, he dabbled in lasers and this resulted in his being sent to boarding school; it is heavily implied in a CBS promotional clip that the government took Sheldon's laser away and hushed the college professor who witnessed the experiment. Sheldon graduated high school very early; he asks Penny, in complete disbelief, "It took you four years to get through high school?!" He told Penny he was eleven years old when he was going to college.

Sheldon also has common qualities associated with being a prodigy, such as an inflated ego, social ineptness, and an inability to identify emotionally with others. He fails to understand not only the simplest sarcastic jokes made by Leonard, but also regards Penny's sadness over her break up with blatant disgust. That isn't to say Sheldon doesn't have a conscience; he's simply more likely to do what's scientifically ethical (not donate to a "high IQ sperm bank" because his sperm won't guarantee high IQ offspring) rather than morally and socially ethical (break into Penny's home to organize her things). Sheldon also remains un-swayed by Penny's beauty. He also has symptoms of obsessive compulsive disorder and Asperger syndrome; when he finds Penny's apartment messy, he sneaks in during the night and cleans it up because he can't stand being in the same hallway with something so disastrous. Also, he refuses to sit anywhere other than his designated spot on the couch. He is calculating and cynical, believing that Leonard is only setting himself up for disappointment as he continues to crush on Penny.

(wikipedia.org)