system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does the scissors-curling of a ribbon you use for a gift work? Are there straight fibers that get broken up in fragments?
gpt
**TL;DR:** *Different amounts of force on the ribbon's outer versus inner edge as you stretch it over your scissor blades results in one surface of the ribbon being stretched out more than the other, and that forms a permanent slight bend that becomes a full curl if long enough.*

A ribbon actually has a little thickness to it. So there's a tiny bit of distance between the side of the ribbon that's facing toward you, and the side of the ribbon that's facing away from you.

When you fold that ribbon over the sharp edge of a pair of scissors and apply pressure, you are actually creating more stretching on the "outside" surface of the ribbon than the "inside" surface. So the outer part of that ribbon gets pulled and stretched more than the inner part, and that combination of a surface that IS stretched and a surface that IS NOT stretched creates a bend. If that bend is long enough, it becomes a curl.

To see this at a larger scale, grab an old paperback magazine and put it in front of you with the staples or seam to the left. Now fold it over right-to-left so the right edge is now touching the spine/seam/staples. Then, holding it in this position, pick it up and look at the U-shaped edge that's closest to you. You can see that the "outer" part of the bend (i.e. the back page) is longer than the inner part of the bend (the front page), and the pages themselves no longer line up squarely, fanning out a little bit thanks to the extra bendy distance. The outer pages are now under more force than the inner near that bend, and they'd stretch more if they could (like a ribbon's material will).

 If that magazine-bending force were strong enough (e.g. put it in a hydraulic press or something), the outer pages would tear before the inner pages.  Same with a ribbon, except its fibres stretch instead of breaking, and you get a curl rather than a rip.