As iron sharpens iron.

The Cooper-Nowitzki Theorem

Penny: I know this is none of my business, but I just... I have to ask — what's Sheldon's deal?

Leonard: What do you mean, "deal"?

Penny: You know, like, what's his deal? Is it girls…? Guys…? Sock puppets…?

Leonard: Honestly, we've been operating under the assumption that he has no deal.

Penny: Come on, everybody has a deal.

Howard: Not Sheldon. Over the years we've formulated a number of theories about how he might reproduce. I'm an advocate of mitosis.

Penny: I'm sorry?

Howard: I believe one day Sheldon will eat an enormous amount of Thai food and split into two Sheldons.

Leonard: On the other hand, I think Sheldon might be the larval form of his species and someday he'll spin a cocoon and emerge two months later with moth wings and an exoskeleton.

Penny: Okay, well, thanks for the nightmares.