This comes from:

• Oxidation of foods
  (Your feel warmer after you have eaten)
• Muscle, liver and other tissue activity
  (internal metabolism)
• Hot foods and drinks
  (Directly transfer heat to body by conduction)
• If there is not enough heat get involuntary muscular contraction - Shivering.This is called thermogenesis (Heat creation). As we get cold blood vessels on the surface of the skin contract so less heat escapes from the skin.

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