"Osmosis?" he asks, supraorbital ridges shifting the way a human's eyebrows might crawl towards their hairline.

"Kid, ain't that above your reading level?" Sans, knowing precisely jack shit about how old human children look, or what an appropriate reading level for them would be, might be taking a low jab at challenging their intelligence, but this is more good-natured ribbing. Besides, rather than just talk about stuff relevant to him, why not give the kid reason to talk about 'em. Even if it's teasing.

"'Sides," he adds, echoing their thought, "That ain't how osmosis works." And then, "You gotta interest in that stuff?"