You lose most of your body heat straight outta the top of your noggin.
For, like, ever, you've been told that if you don't wear a hat, you'll freeze for sure. Now, it turns out that's a bunch of hot air. Any part of your body exposed to the elements--an ungloved hand, a bare neck--loses heat, according to researcher Daniel I. Sessler, University of Louisville School of Medicine. If you're running around in a swimsuit during a snowstorm, your head loses as much heat as the rest of your exposed bod. So ... wrap up, like a hot burrito.