When Amy's (Mayim Bialik) apartment floods and she needs a place to stay for five weeks until it's repaired, Penny (Kaley Cuoco) suggests that Amy move into Leonard's (Johnny Galecki) room in 4A, allowing Leonard to fully move into 4B with his wife. Finally, the Hofstadters will be free of their burdensome third wheel.
Amy proposes this arrangement to Sheldon as a scientific experiment in cohabitation, but Sheldon decides he'll only do it if they get their own place, because he doesn't want the comfort of his apartment as a variable. So it's decided that Amy and Sheldon will move across the hall into Penny's apartment, while Leonard and Penny stay in 4A.
On the Hofstadters' first night alone, once Leonard is sure Sheldon is asleep and won't be barging in, he has an idea of something he and Penny can do now that they never could have done while Sheldon was there: blast music and dance in their underwear.
"Ugh, can we just have sex?" Penny groans.
"Don't worry, once you see my sweet moves, sex is inevitable," quips Leonard.
And honestly, his moves are pretty sweet:
Leonard's hips don't lie.
The song is "You Never Can Tell" by Chuck Berry, aka "the song John Travolta and Uma Thurman dance to in that famous scene from Pulp Fiction," which is why Leonard does this Travolta-referencing move:
The song that convinces Penny to join Leonard is "Tootsee Roll" by 69 Boyz.
The Big Bang Theory airs Thursdays at 8/7c on CBS.
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