"in the paper making process, the pulp is strained to create the initial textured/brittle paper, but the process doesn't end there. The textured paper must be put through high pressured steam rollers to create the smooth paper you've come to know. This setting process causes all the fibers to be pressed flat and packed together evenly on the surface. When paper gets wet, it "unsets" itself, breaking up the tension that held the fibers together, again causing the uneven texture it initially had." Got this from reddit
