system
Given the following post, respond as if you're commenting on Reddit:
human
What happens, on a molecular level, that leads to paper becoming soft after applying water to it?
gpt
Paper consists (mostly) of cellulose fibers, i.e. a polymer of glucose molecules. These glucose molecules have free hydroxyl groups, which can form hydrogen bonds to other molecules. 

In dry paper, at least many of these hydrogen bonds are to neighbouring cellulose fibers, which stabilises the overall structure.

Water breaks up the hydrogen bonds between the cellulose fibers, so this stabilisation is lost.