As you read these words, the air’s molecules are ­zipping around you at 2,000 miles per hour, faster than a speeding bullet, and bombarding you from all sides. Meanwhile the atoms and molecules that make up your body incessantly tumble, vibrate or collide with one another. Nothing in nature is ever perfectly still, and the faster things go, the more energy they carry; the ­collective energy of atoms and molecules is what we call, and feel as, heat.