The storm had soaked them all to the point where they needed something clean, dry and fresh. Luckily the depot they were using as a shelter had plenty.
Sans, Frisk, and Papyrus each excused themselves to undress and at least put some pants on [Frisk insisted on privacy to pull on an oversized, striped turtleneck] and then wrestled with whatever topwear they had found.
Sans, of course, was never happier than when he was in a hoodie. The plain fleece cloth draped over a belly that wasn't visible when he had his top off.
Papyrus struggled into what seemed to be an oversized, long-sleeved shirt. And when he finally had it on, an immense bulk of sculpted muscle stretched it to capacity.
Frisk pointed. They were used to seeing Sans' invisible gut, but this boggled their mind.
"What?" said Sans. "Oh yeah. Ghost-flesh. Skeletons are sorta related to ghosts. Only the bones are the real part of us. The rest is invisible in us, but visible with ghosts. It's... kinda weird. Monster clothing can ignore Ghost-flesh, but human clothes? No such luck."
Papyrus had found a mirror. "Nyeh heh heh! All that training really paid off. I should wear human clothing more often."