system
Given the following post, respond as if you're commenting on Reddit:
human
Why is it easy to iron a wrinkle into a shirt, but not to flatten it back out?

Surely you've been ironing a shirt or something and accidentally ironed a wrinkle or crease in the fabric where it doesn't belong only to find it surprisingly stubborn to flatten out. So why is that?
gpt
The best way to imagine a shirt is like a piece of paper. And the best way to imagine paper is to imagine a shirt. Both are made up by layer upon layer upon layer of strands of material that are bound together. So, to prevent a circular loop, imagine a whole bunch of straws layered together. 

What happens when you fold it? Well, If you have a red side and a blue side and fold it so that the blue side ends up touching itself (gross), the red side has to expand because what was a flat surface is now expanded to allow for a curve (the blue side doesn't shrink and everything has to go somewhere, so it forces the red surface to wrap around it and expand). It expands out by stretching, but because the straws themselves don't stretch, they slide out of place and the red side grows. This stretching is tension caused by compression of the folded material, but is tension across the red surface.

When you unfold it, the red side is now bigger and you get that bump because that extra surface area has to go somewhere (the crease is also held in place by the smaller blue side as well). 

When you try to iron it back in place, the blue side is not under enough tension as the red side was during the folding and you can't shrink the red side by pressing down on it with an iron.

Why does water work? Because it does two things, it causes everything to swell up and it lubricates the fibers (also, hot water is also absorbed by animal hair and causes it to swell and relax, that's why showers are good for bedhead). The blue surface swells up to be almost as big as the red surface and the crease is mostly gone just from a good soaking. The relaxed and lubricated fibers of the blue surface can now expand to the same size as the red surface and the crease is mostly eliminated when ironed. There is some shrinking during drying, but some shirts also stretch out over time.