Kites are sort of new to monsters. The idea of flying a thin object under some wind wasn't a bizarre concept, but they didn't have much wind under a mountain to begin with. Not real wind, anyway. So when Sans catches the red object out of the corner of his eye, curiosity gets the best of him and he follows it.

Thinking about it now, Sans hadn't been out to the edge much either. By the time he finds Frisk, he's mostly staring between the ground... and then the kite again.

"Hey, bud."

He nods up.

"That's called a... kite, right?"