Why does wet paper curl?

Question

I mean, once it's wet and dries, it's wrinkled and curls in the corners, and nothing you can do can flatten it out short of ironing it.

By the way, what does the ironing do to straighten/flatten it?

Answer 1

wet paper curl because the paper absorbed the water and then air in the surroundings tries to evaporate the water which in turn makes its surface irregular.

Answer 2

when paper becomes wet, the molecules in paper tend to attract each other, which results in curling.