Sheldon hides in stealth in the crowd during Leonard's presentation. Very incognito.
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Penny falls asleep when she, Wolowitz and Raj go to listen to Leonard give a presentation.
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Sheldon tries to attack Leonard with his mind when he goes to give a presentation that Sheldon does not approve of.
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The Big Bang Theory Season 1 Episode 9 Quotes

Leonard: Sheldon, why is this letter in the trash?
Sheldon: Well, there's always the possibility that a trash can spontaneously formed around the letter, but Occam's Razor would suggest that someone threw it out

Leonard: You are not Isaac Newton.
Sheldon: No, no, that's true. Gravity would have been apparent to me without the apple