Leonard: Well, if Sheldon and Amy don't want to expose Pemberton, that doesn't mean someone else can't do it for them. They'd never have to know.
Penny: Really? You'd do that?
Leonard: They deserve the Nobel. I'm not gonna let two frauds steal it from them. And if that means getting my hands dirty, so be it.
[Penny laughs]
Leonard: What's so funny?
Penny: Just realized I've never actually seen your hands dirty.