Shivering occurs when many muscles contract a little bit all at once.
Heat production is increased by shivering and non-shivering thermogenesis .
Heat production is increased when many muscles contract a little bit, all at once.
When many muscles contract a little bit, simultaenous, it produces:
  * forces
  * heat
  * cold blooded
  * sweat
  * electrical energy
  * Energy
  * pain
  * thermal displacement