It all came down to oxygen: much the way you build a fire by blowing on it, you can crank up your internal furnace by sucking in air. That’s why you gasp and shriek when someone pushes you into the pool; the cold shock triggers your lungs into hyperdrive, accelerating your circulatory system. Super-oxygenating your blood not only warms you up, but calms you down; and because a clear head helps you survive in dire straits, your brain quickly releases soothing hormones to help you (ahem) chill out.
