Re: When paper gets wet, why does it dry wrinkly instead of flat?

Paper is made of cellulose fibers. In many ways it is similar to a cotton 
shirt.  When you wash a cotton shirt it usually is wrinkled until 
you iron it.  Paper is similar.  Paper making machines press the cellulose 
pulp into sheets.  As it dries, the fibers stick together.  If the fibers dry 
in a more random arrangement, wrinkling will result.  Rewetting and pressing 
will eliminate the wrinkles (take care not to scorch the paper).

An interesting reference on the discovery of cellulose: 
http://www.afandpa.org/products/products.html