Your brain cranks up the heat in two main ways: For one, it may generate more heat by inducing shivering, which is just your muscles contracting really quickly, which generates heat as a byproduct. And by making it harder for this heat to escape by contracting the blood vessels close to the surface of your body, which reduces the heat that can escape through your skin. This is also the reason why you can feel so cold when you have a fever—your skin is actually colder because your body is trying to really heat up your core and create unpleasant temperatures at the battlefield to make pathogens
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