system
Given the following post, respond as if you're commenting on Reddit:
human
What actually is happening when you fold paper? What makes it go from bending to creasing at a certain point?

The same can apply to other materials, but I fold paper all day at work. Which reminds me, I should get back to work.
gpt
A lot of materials have a yield stress. Including paper. When enough force is applied, the stress in the paper can exceed its yield stress. From that point on the material is plasticly deformed, which means when you stop applying force,  the paper will not bend back into its original shape.