Sherlock and William are sitting together on the train. They were having a conversation about the "Napoleon of crime" and the recent case of earl Drebber. Sherlock then got an idea and decided to bluff.

Sherlock: The murder of Drebber, it secretly weighed on Hope's conscience. He secretly told me the name of that culprit.

There's a moment of silence between the two and Sherlock looks at William with a smirk.

Sherlock: So you were "that person". William James Moriarty.

William stayed silent for a moment, then smirked.

William: That's an amusing accusation, but impossible to prove.

Sherlock laughs

Sherlock: It's a joke, a joke. I would've liked you to be that sharp criminal.

William: Catch me if you can, mister Holmes.