system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What makes a ribbon curl when I slide it over the sharp edge of a scissor?
gpt
When you bend the ribbon over a sharp edge, it stretches the outside of the ribbon - permanently making the outside a little bit longer. Dragging the ribbon along the sharp edge is like bending it a little bit at every point. This makes the outside of the ribbon slightly longer than the inside, forcing it to curl. Because it's stretched evenly along its length, rather than at one point, you get a smooth curl rather than a sharp bend.