I take a thick book, open it to the middle, put a pencil in it, and then close the book. If I then wait five minutes and take the pencil out, the book will go back to normal. If I instead wait five weeks and take the pencil out, the book won't close all the way- there will be a small gap. Why does this happen?
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The interlocking cellulose fibers that make up paper spring back upon being stressed and quickly released, but at the same time they are also capable of slowly slipping past one another under stresses that are applied over long times. This is called creep in metals and taking a set in nonmetals. In paper, moisture helps unlock the fibers, reducing the wet strength of the sheet, and binders (glues mixed into the fiber structure) increase the wet strength and somewhat reduce the paper's tendency to take a set when bent or folded.
niels nielsen
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