That’s because we all have a certain amount of brown fat cells – and they can sense the cold. The cold triggers them to start warming us up – that’s why we have them – those brown fat cells release their energy as heat. And when they’re activated, we torch calories and melt our other fat, the white fat. So bundle up and take your workout outdoors. Just remember to protect your limbs, fingers and toes – which have a harder time staying warm.