Sheldon: You're not done with her, are you?
Leonard: Our babies will be smart AND beautiful.
Sheldon: Not to mention imaginary.
While the group are trying to figure out how to play Halo without Howard, Leonard jokingly suggests cutting Raj in half. Raj responds, "Oh sure, cut the foreigner in half. There's a billion more where he came from." But Penny is in the room with them, so there's no way he should be able to talk in front of her, or even if he's been drinking, the fact alcohol helps him isn't realised until the next episode, so the fact he can speak with her there should have surprised everyone, including him.
After Sheldon mentions a molecular sieve, they go over to 4a, their apartment. 4a is a kind of molecular sieve.