Fainting is no joke. Every year, it sends four million people to the ER, according to the Wall Street Journal, especially if they fall after blacking out. That light-headed feeling you get comes about when there's a drop in blood flow and oxygen to your brain--and there are a number of reasons that could happen. You could be suffering from shock, from the sight of blood, say, or hearing bad news; your blood sugar levels could have dropped from not eating or drinking for a few hours; or your blood pressure could have dropped because you stood up too quickly (common among people over 65).
If you're feeling dizzy, try this trick, courtesy of a new study: sniff or gasp, with your mouth closed, twice. The quick intake of breath can send oxygen rushing to your brain, which relieves that light-headed feeling. You can also try raising your blood pressure by crossing your legs tightly or making fists with your hands, a technique known as applying counter-pressure.