Maybe it's a bride standing in a hot chapel, or an exhausted runner after a race. It could be someone watching a medical procedure on television or a donor at a blood drive.
Maybe you've even experienced it yourself. You start to feel lightheaded, your stomach may hurt, your palms are sweaty, your vision closes in, your ears start to ring ... Then you wake up on the floor, staring up the ceiling, and realize you've fainted. What happened?
Fainting — or what medics more technically call syncope — can be caused by a number of factors. Ultimately it comes down to not enough blood getting to your brain.