What, did Derek lose a bet to Stiles?
Yes, yes he did. It’s Undie Run, the Wednesday of finals week at BHU, and the terms of the bet specifically was that Derek do the entire run in whatever Stiles picks out. Now, Stiles could have been extremely cruel and gave him just a sock or a man thong, but he figures that Associated Students President, all around goody-goody basketball jock Derek Hale would never live down the ultimate nerdery: glow-in-the-dark Star Wars boxer briefs with the words “ASK ME ABOUT MY LIGHTSABER” emblazoned on the front, right over the crotch.
Okay, so maybe this wasn’t the best of Stiles’ ideas, watching Derek step out of the bathroom wearing only the briefs— and Stiles had gone a size down, just for fun, but he didn’t think they’d be that tight, and wow, he definitely wants to know more about that bulge—
Derek flexes mockingly in front of Stiles, and Stiles’ throat goes dry. “This good enough for you, Stilinski?”
"Yeah, that’s— that’s good," Stiles stammers, and Derek gives him a grim salute before heading out to join the rest of the drunken crowd ready to race across campus.
Stiles normally hates Undie Run; it’s loud and obnoxious, but this time he’s got invested stakes so he watches carefully from his dorm window. He easily picks out Derek’s form, jogging with determination, veering clear of the crowd. Derek doesn’t actually seem embarrassed at all, he’s just…
Well, at least Stiles got a good show out of it.
He flops back down on his bed, starting up a new game of Skyrim when there’s a rapping at his door. Stiles opens it, and is surprised to see Derek standing in the hallway, a slight sheen of sweat on his bare chest, still wearing the underwear and holding a brown paper bag.
"Well?" Derek asks.
"What?" Stiles responds, confused.
"You went to all this trouble to get me in this getup, and you didn’t even ask," Derek says, smirking.
"Ask, um, what?"
Derek looks down at himself pointedly, and then says, “I also bought glow-in-the-dark condoms, if you’re into that.”
"Into that I am,” Stiles says delightedly.