When I spill water on paper and then the paper dries/the water evaporates, the paper is extremely stiff and crinkly (makes noise when moved).
Why is this? I understand that water breaks hydrogen bonds in between cellulose fibers in paper making it easy to rip, but when it dries, why is it stronger?
Great question. Wetting leads to a collapse of the cellulose fiber network. When dried, the deformed, "squashed" fibers are in greater contact than they were pre-wetting, and hence give rise to a dense, stiff mesh. A more thorough discussion is in this paper:
"Why does paper get stronger as it dries?" Alvaro Tejado and Theo G.M.van de Ven, Materials Today, 13, September 2010, Pages 42-49.
