AU where Frisk is a mute 6 or 7-year-old when they reach the Underworld.
Sans and Papyrus don’t need to sleep or eat as regularly as everyone else but they do it anyway.
They don’t need to drink water at all.
2 days after meeting Frisk they collapse and the skelebros are desperately trying to figure out why. There are no doctors in snowdin- everyone just treats each other. But who can they go to for a human child? Frisk is declining fast that night and sans goes out to Grillbys for a bit. He doesn’t say much but the bartender seems to know something’s going on anyway.
He’s not gone long, getting a to-go kind of meal before leaving. Later that night- just before 12 AM- there’s a knock on the door and when Pap opens it there’s Grillsby with about 12 oven-mitts over his hands, pinching a large mug of water between them. Papyrus is confused but takes the glass and the second Frisk sees it they grab it and chug it all in one go and then get sick and puke all over the place.
And that’s how the skelebros learn humans need water every day to live.
Pap later learns, through yet more trial and error, that, though Frisk needs water to live, they also need oxygen.
Toriel finds out much later and they’re all just kinda like
