Why do football players paint black marks on their cheeks?
It prevents distracting glare from sweaty cheeks. You might think it odd that football players do this, while many other athletes do not. This has to do with the fact that the head is inside a helmet, and the eyes become accustomed to the relative darkness, so sudden flashes of light reflecting off of the cheeks can be disproportionately distracting. You may have also noticed baseball players doing the same thing for similar reasons; though the eye altering darkness comes from the baseball cap instead of a football helmet.
