Penny & Sheldon The weird comic book crossover

This is cute, but I don't get why Sheldon is the Hulk?

Cause it refers to an episode. Penny gets addicted to video games and asks Sheldon's help to move rounds, the rest of the guys are seeing them interact with one another and that's when Raj and Howard say those lines: "It's like a weird comic book crossover, as if Hulk were dating Peppermint Patty.."http://www.youtube.com/wattch­?v=­zix­lt3­zia­IY&­amp­;fe­atu­re=­pla­yer­_em­bed­ded