Penny comes to Sheldon for help to better understand Leonard's work and here he is training her.
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Bernadette (Melissa Rauch) joins the guys for lunch at work as she begins to bond with Leonard over his work.
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Wolowitz confronts Leonard when he gets jealous over his friend bonding with his girlfriend, Bernadette.
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Raj and Sheldon race... with a little Mario Kart on the Wii.
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Wolowitz gets jealous when Leonard and his girlfriend Bernadette (Melissa Rauch) get friendly.
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The Big Bang Theory Season 3 Episode 10 Quotes

Sheldon: Why are you crying?
Penny: Because I'm stupid!
Sheldon: Well that's no reason to cry; one cries because one is sad. For example, I cry because others are stupid and it makes me sad.

Sheldon: Where's your notebook?
Penny: I don't have one.
Sheldon: How are you gonna take notes without a notebook?
Penny: I have to take notes?
Sheldon: How else are you gonna study for the tests?
Penny: There's gonna be a test?
Sheldon: Tests. Here, it's college ruled, I hope that's not too intimidating.