Why does paper under pressure flatten over time? If I have a crumpled paper I can put it under a heavy book. If I remove the book in a minute the paper will still be rather crumpled, but if I leave it on for a longer time it will flatten more. 
But why? There is no movement involved, so where does the energy to flatten the paper come from?
 A: "Practical Considerations for Humidifying and Flattening Paper" (2003) by Stephanie Watkins, a conservationist, has this to say about how to flatten paper (emphasis mine):

The aim of humidification is to reintroduce moisture into the paper support to relax the fibers... Gravity and time, or
  pressure and time, can be as effective, depending on the
  relative humidity of the storage area. Curled paper that is
  sturdy can be hung from flat clips, such as paper-protected
  bull clips, and left over a short time to slowly uncurl
  (e.g. panoramas, large blueprints, etc.). Protect the items
  from dust and light exposure during this process as it may
  take several weeks.
  However, humidification relaxes the paper in a faster
  manner and fibers are less likely to be stressed.

Thus:


*

*Paper becomes flat once the microscopic cellulose fibers in the paper relax.

*This process does not complete instantaneously upon application of a force, i.e. it takes time so the longer you leave it the flatter it becomes.

*The energy should be coming from the book lowering ever so slightly as the creases in the paper disappear.  

