If Sans hadn't been a physicist, he might've entered the realm of biology instead.

It's the idea of different textures that appeals to him. The different shapes of matter, magic formed to mimic the living, possessing muscles, tendons, organs and so on. Skeletons sort of skipped over most of that, retaining the structure of bones, hard and unyielding, but still susceptible to fractures. No flesh cushion to soften a blow, or to allow a hand to feel nice instead of just sharp and edged.

If he had studied biology, he might have had answers to Toriel's fur, the soft cartilage between her fingers, the play of light and moisture over her eyes. It's the opposite of everything that Sans was, and that was fascinating. Incredibly, and utterly fascinating. Yeah, maybe that was why he never let himself get into the field. He might have started getting into it too much. And those kind of researchers were always the freaks of the bunch.

He stretched his creaking bones, blinking away nothing. There was no biological need for him to recharge his physical reserves (as Papyrus' non-existent sleeping schedule demonstrated so well), not unless he could manipulate his magic to do so. Which was possible, and easier during the day. But not when it was dark, and not when the knowledge of an expansive world waited outside his walls.

He had long grown used to the room's darkness; to the shadows that outlined the dresser, the small portrait on the wall, and the diary that lay perched on the small desk by the right wall. Easy to note, especially on those nights when sleep just wasn't coming, yet still too tired for a shortcut, and needing to maneuver around the deep pitch to get a tall glass of milk from the kitchen.

He made to get up from the bed, but she had felt him then.

"Sans?" Toriel called to him, the bed sheets over her form. She blinked, lifted up her head, her fur parting from each and every miniscule movement.

He remembered the first time he had indulged in what she was. The shadows had done nothing to hide her; they had only heightened the very weight of her form, how she had made a slight dip in the mattress, and how her eyes, not nearly as bright as his, could honestly be felt more than seen. His own bones were hollow and weightless – he could stand on the bed and there would be no hint of a spring creaking from the motion. But he had fallen against her, drowning out every knowledge of his own self so that he could know her instead, so that he could exchange something much more valuable and interesting, composed of blood and magic and intensely corporeal in a number of ways. She had pressed softly-padded hands against his bones like gentle things, like precious things. He couldn't blame her – bones woven by magic, especially his, can be so intensely fragile. But her smile, from that night and all nights following after, spoke of other things, and he was still learning to translate those movements of flesh into something that even a numbskull like him could understand.

"Is everything alright?" she asked him with eternal patience.

Sans gently pushed aside the covers, allowed one skeletal hand to press against her side, to watch his fingers completely drown in soft pelt, to be engulfed by heat. Another hand reached around her head, thumb brushing against one of her horns, the only part that was anything like this sorry bag of bones. He tried to see what she must be seeing right now, but he was never that generous with himself. He made due with her smile, and trusted in what it believed. He leaned forward to feel that smile against his own.

"never better, t," he said, meaning it.