It was a comfortable domestic scene, despite the fact that it was happening inside a mansion. Sans lounged and dozed on the softest couch by the tepid fire as Toriel carefully brushed and combed Frisk's hair.
Frisk was loving it, Sans could tell. The way they leaned into every single caring touch was a dead give-away.
And then Toriel said it. "I think it shall be time for a haircut, soon."
Frisk nodded agreeably.
"Er," said Sans. Suddenly awake and rigidly upright. "A what cut?"
Toriel smiled. "A haircut. Parts of humans grow at astonishing rates, as I recall."
Sans was already on his phone. Calling his brother. "You are not gonna believe this..."
Which lead to two astonished skeletons staring at everything in the nearest hairdressers. At least one of them was taking frenetic notes. Frisk wasn't even paying attention, most of the time, as shiny blades came close to their head. They seemed pre-occupied with something inside their mouth.
"Is something the matter, my child?" prompted Toriel, who was getting her fur primped in solidarity.
Frisk reached into their mouth and, after a little work, pulled out a slightly bloody tooth. They grinned in triumph.
There was a moment of breathless silence from the skeletons. Then a clattering noise as they both hit the tiled floor. Fainted dead away, as it were.
"Oh dear," sighed Toriel.
Frisk was never much for loud noise at all. Their loudest giggle or shriek never got much above the volume of more normal human speaking. None of the experts could say anything more about it than that Frisk was a naturally quiet kid.
Which was why The Sneeze scared the hell out of everyone.
They were minding their own business on a mild Autumnal afternoon. Just hanging out and doing nothing very much. Frisk was seemingly pulling strange and twitching faces as they watched the squirrels preparing for the turning seasons.
And then, without any warning at all...
It was the loudest noise to ever come out of a human. Even the loud humans who had no volume control below "shout".
"...the hell?" mumbled Sans.
Papyrus was already running for Undyne. Screaming for his life.
Frisk had time to sign, "It's okay," before it happened again. "HASHOOUFF! HASHOOUFF! HASHOOUFF!" Each explosion of noise was accompanied by rapid, jerking motions, and attempts to block the noise with a sleeve.
"Goodness, those are loud sneezes," cooed Toriel. Coming to the rescue with a box of tissues, a beannie, and a super-large, fluffy, striped coat. She automatically helped Frisk into the garments. "This is not Snowdin, where the cold will not harm you," she chided. "You have to keep warm or you might catch something."
Somewhere in the depths of the coat and beannie combo, Frisk blew their nose. A comparatively small hand emerged to sign, "Sorry."
"What even was that?" said Sans.
Toriel was checking Frisk's temperature with a massive paw over their cheek. Reading their stats with a worried gaze. "They will be fine," she sighed in relief. "It's just a minor irritation. Cold weather and spores."
Frisk nodded. Their sleeve tried to pat Toriel's arm reassuringly.
"Yeah, but what was it?"
Frisk made an unfamiliar sign.
"Sneezing," said Toriel. "One of the many human defences against some of the little things that could... harm them. Sometimes, it means they are getting... sick..." Ancient hurt glinted in her eyes for a moment. She took both Sans' hand and Frisk's in hers. "Come now. It is time to be inside."
Sans' grip was a little tighter than it needed to be. He needed Toriel's reassuring presence, right now. "Anything else that humans do that I should be warned about?"