Eren absently picked the wedgie out of his ass, tugging the fabric back over his buttcheek. He glanced from the TV to the clock, a bit concerned. Normally he'd have heard Armin's car come up the driveway by now, and he'd run out to greet him. These new aerodynamic action shorts that Reiner got him interested in really helped him improve on his running speed. From the way Armin kept accidentally dropping things and having to ask Eren to bend over and pick it up for him, maybe Armin could use a pair, too. They were very easy to move in, after all.
There was a knock upon the door. Eren straightened the knot on his gingham tie-off shirt, and trotted to go answer. He opened the door to find Mikasa, staring at him evenly, a mysterious bag held at her side. Eren was immediately set on guard.
"Armin's judgment has been compromised, so he is unable to join me for this intervention," Mikasa said, solemn. She held the bag in Eren's face. "You will change into real clothes immediately or face the consequences."
Eren would give up his action shorts over his own dead body. Barking a quick, petulant "no", Eren raced past her out the door, and down the sidewalk. He heard the oncoming storm of Mikasa's racing feet behind him.
Action shorts, don't fail him now.
(They failed him. The action shorts remain, to this day, mounted upon Mikasa's mantelpiece as a trophy, a warning. Every year do Eren and Armin both mourn their passing.)