Why does paper crease when it's folded?

Question

just curious

Answer 1

weird question lol

Answer 2

Some materials hold thier shape better than others.  Some of the factors that determine this is the thickness of the material, and it's make up.

In the case of paper, it is made up of fibers which consist of, among other things, wood pulp and fibers made from cloth.  It is the wood pulp fibers that let a sheet of paper hold it's shape, instead of, say, a cloth napkin, which would, simply, hang and drape.

So, it is this same fiber that allows a piece of paper to remain stiff, that makes the same piece of paper hold a crease when it is folded.  Some of the wood pulp bits fold, and, even, break, when the paper is folded, so, that when unfolded, the damaged fibers and bits show the damage, showing the crease.

Some cloth, for example, may not show a crease when folded and unfolded.  Other materials, like thin sheets of cellophane may also not show a crease, until pressed into place, and the outside surface gets stretched farther than the inside surface.