system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it that a wet tissue or piece of cardboard or paper tear more easily? How does water affect the structural integrity of paper products?
gpt
Paper products are made from wood fibers that are basically matted together in random orientations like felt. As the paper dries, little hairs and random kinks in the fibers cause them to link up like tangled hair. Pressing/rolling operations enhance this effect.

Paper fiber is very hydrophilic, meaning it can soak up water easily and will soak up far more than its own weight in water. As the water wets the paper, capillary action pulls the water into the spaces between fibers and forces them apart. At the same same time, the fibers themselves absorb water, swelling up.

Between the swelling and the capillary action, the fibers are simply not as able to cling together.