Why do your teeth chatter when your cold?

Answer 1

Teeth chattering is just a symptom of shivering. Endotherms (that's you, me, other mammals, birds, and some insects) produce heat within the body. They use thermoregulation to keep their bodies at a constant temperature. You can thermoregulate physiologically and behaviourally. Shivering is a physiological response to cold, it's coming from inside. Putting on a jacket because you're cold is a behavioral response.

Let's say you're cold and you won't put on a jacket, or do a lap around the block to keep warm. Your brain tells your body what it needs to do. The brain -- the hypothalamus, to be specific -- monitors your temperature. Get too cold and and the hypothalamus sends nerve impulses to the skin and you get goose bumps. The goose bumps are caused by teeny muscles attached to the base of your hair follicles. Since humans lack a tonne of body hair, the goose bumps do little to make us warmer. But for furry animals those little muscles create better insulation. The muscles raise the hairs and the hairs trap air next to the skin. Because air is an insulator, this air next to the skin acts like a blanket to keep an animal warm.