Say I have a piece of paper that I fold in half 8 times 
Now when I unfold it there are going to be creases on the paper. I can try and straighten it out by moving it over the corner of a desk or even a steam roller but the creases, or at least wrinkles will still be there. When you fold something like a piece of paper does it rearrange the molecules to change its shape? How is it so easy to crease soemthing but so hard to get it to go away?
In metal it would be a plastic deformation, I imagine it works the same for paper; you can bend (deform) it up to a certain point, after that the deformation is permanent.
I think it has to do with breaking down the fibers in the paper by flexing them through compression (inside of the fold) and tension (outside of the fold). A crease just shows where you have distorted or broke down those fibers and reduced the papers (rigidness?)
If you make enough folds in paper over and over (crunch it up in a ball, flatten it out, crunch it up in a ball, flatten it out, repeat) the fibers break down enough where the paper becomes soft and losses all rigidness.
Metals (and by this I mean the room temperature elastic-ductile elements and alloys that we’re familiar with, like steel, aluminum, copper, tin, zinc, silver, gold, platinum, et cetera) behave quite differently from non-metallic compounds like polymers or ceramics. The mechanisms by which metals deform cover a range of different behaviors but essential comes from the fact that metals have a homogeneous long-range structure. By homogeneous we mean that the lattices that metallic atoms form are identical along their lattice directions, and by long range we mean the span of thousands or millions of atoms, which form a “grain” (an agglomeration of millions of which makes up a metal part) that is essentially one big molecule. When you place these under stress, the lattice “gives” somewhat , then pops back into place once the load is relieved; this is elastic deformation. Under larger forces, the matrix will realign into a different form and/or grains will change their orientation with one another, giving you plastic (permanent) deformation. (My apologies to metallurgists who are probably having apoplyptic fits with my highly simplifed version of metallurgical deformation, but such is life in the fast lane.)
With polymers and fiberous composites (of which paper is one) deformation is different. Polymers (long molecule chains) have a tendenancy to twist in different directions, and under stress untwist or twist in different ways. Twist them to far and they’ll start shifting with respect to one another, or even break apart. On the larger level, the fibers of a composite are held together by a matrix. For fiberglass it is the resin in which the short fibers are mixed. For a woven fabric, it’s basically the friction/electrostatic attraction between the fiber layers. For paper, it’s somewhat in between, with the wood pulp being mixed, smashed, stretched, flattened, and dried into its final form. The paper is held together, and largely comprised of a cement of cellulose carbohydrates that bind together the larger-scale cellulose fibers you see sticking out of the edge of a piece of paper when you tear it. What causes the creasing is that this cement is breaking, leaving only the longer fibers to hold things together, similiar to breaking a piece of drywall and having only the paper backing keeping the fragments in place.
And to think all of that is happening when you fold a letter so you can mail a question to Cecil. If you stop and ponder all the enzymatic reactions and chemical decompositions that are occuring when you lick the adhesive flap, you’ll never make it to the post before collection time. It’s a good thing they have self-adhesive stamps now; you don’t need to think about those at all, since they work by utter magic.
Stranger
au contraire - self adhesive stamps are a wonderful mixture of polymer chemistry, solubilization, viscoelasticity and adhesives forces.
zippers on the other hand - black wizardry