On the [Main Menu] screen, push [Up] or [Down] on your control to highlight "Hear Tappy Tibbons' Amazing Life Story!" to see a special infomercial that Darren Aronofsky shot as backstory to the Tappy Tibbons character.
Go to the [Chapter Selection] screen. Highlight the bar for chapter 21-24 (at the bottom it says '#3 shh'. Press the [Up] button on your dvd remote twice and then press [Enter]. A secret portion of the infomercial will play and "Number 3" will finally be revealed.
