Whenever I go skiing and I'm riding up the chairlift, it gets really cold with the wind blowing, and my teeth always chatter and my body quivers. And it's totally out of my control. What's up with that?
The brain - the hypothalamus, to be specific - monitors your temperature. Get too cold and the hypothalamus sends nerve impulses to the skin and you get goose bumps, which do very little to make us warmer. When furry critters get goosebumps, the hairs lift up and trap air next to their skin, which acts like a blanket to keep them warm.
We're hardly furry enough to use air as a blanket, so our bodies came up with a different strategy to regulate our body temperature. When you get cold, your muscles get a signal to start contracting - that's the shivering and teeth chattering. The by-product of the muscle contractions is heat.
So the next time you start shivering and chattering, you can thank your body for doing its job to keep you warm.