Stand outside a Flywheel or CrossFit studio, and a stream of sweaty, red-hued faces will eventually stagger out. You may have noticed the same unsubtle rouging of your own cheeks after a long run or bike ride. But why so red? “As your body heats up during activity, your core temperature and your skin temperature increase,” says Edward Coyle, director of the Human Performance Laboratory at the University of Texas at Austin.