You and Bart always bet, why? Because it's fun, so why not? Last time, it was him to lose and you made him wear a stupid chicken costume and walk around the Watchtower like that—it was embarrassing? Yeah. But it was kind of funny seeing the other heroes reactions.
Still, this time, it was your turn to lose. Maybe betting on something that he already knew the results of was kind of a dick move from him, but you don't need to know that part, he simply wants a little revenge. He had the perfect idea for a ridiculous costume...
When you got out of your room, Bart bursted out laughing. "HAHAHAHAHA! Oh man! I can't--" he kept laughing for a whole minute as he watched you wearing the costume of Open-Window Man. "So, how do you feel?" Bart asked you between a few giggles, he knows what your answer is gonna be though. Who would feel like they look good in that thing except for Jed Oliver?