system
Given the following post, respond as if you're commenting on Reddit:
human
Why does something straight, like ribbon or hair, curl when you drag scissors along its surface?
gpt
With ribbon this happens when you hold a ribbon taut and drag a sharp object along the surface of the ribbon. The pressure exerted by the sharp object forms a shallow depression in the ribbon, and since the inner surface of any concave curve is shorter than the outer surface of the curve, the ribbon stretches more on the outer surface than the inner surface.

This doesn't work as well when you attempt it with the ribbon on a flat surface because the ribbon is less able to form a depression. It also doesn't work when you attempt it with many other materials because they're unable to undergo plastic deformation (deformation in which the material doesn't return to its original shape).