Sans's grin seems to widen a bit when Brainiac asks and then immediately takes back his question, because yes, the answer is magic, and it's always pretty amusing to use that to explain things to people who hate that kind of response. But, seeing as Brainiac seems to have already exasperated himself just thinking about it, Sans counts that as a job done. It's very convenient that way.

"There's still a pretty good chance this'd go right through me," he says instead, turning over the whatever-this-is in his hands. "It seems pretty solid, y'know? I might stain my good clothes."

These clothes don't look at all dressy, except for the rumpled tie.