The first four episodes of The Big Bang Theory portray or allude to Sheldon doing things that are inconsistent with his later characterization, including trying to impress Penny, knocking at doors in a ”normal” way, participating in a salsa class, drinking a beer, and attempting to donate to a sperm bank (though he eventually refuses as he feels there is no guarantee his sperm will produce high-IQ offspring). According to Prady, the character ”began to evolve after episode five or so and became his own thing”.
Aside from his idiosyncrasies, Sheldon is logical. He possesses an eidetic memory and an IQ of 187, although he claims his IQ cannot be accurately measured by normal tests. He has claimed to have a Masters Degree and two doctoral degrees in early episodes. However, by the episode ”The Love Car Displacement”, his list of qualifications had increased. Sheldon has an overly extensive general knowledge, such as shown by his comments regarding various details of anecdotal knowledge (for instance, about the introduction of the fork into Thailand).
Despite his intelligence, Sheldon is usually inept in most social interactions. His eccentricities, direct remarks, and demanding nature put him at odds with his own friends and especially Penny. He has a distinct lack of emotional maturity and is often baffled by even the most common social interactions. He not only fails to understand the simplest sarcastic jokes made by Leonard, but also regards Penny’s sadness over one of her breakups with blatant confusion. However, recently, he has begun to understand the concept of sarcasm, attempting unsuccessfully to employ it himself towards Penny in the second season, and successfully employing it towards his friends in the third season. Sheldon occasionally uses slang (in a very unnatural fashion), and follows jokes with his catchphrase ”Bazinga!”.