Every cell in your body has those things called mitochondria. Every single one of those works basically like a furnace. They burn fuel (usually glucose) in the presence of oxygen (which you breathe in) and generate energy and heat. Multiply this by the number of cells you have and add the fact that your body can control how much oxygen it gives to its cells and you get a body that generates heat to keep itself at a cozy 37 degrees Celsius.
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