If there's spurting, like in a B-horror flick, you've hit an artery and need an ambulance—now. While you wait, slow the bleeding with a tourniquet; loop a belt or cloth around the limb and twist it tight with a stick to clamp off the blood supply. If there's no spurting, apply pressure to the wound. Don't worry about searching for the cleanest possible material—just grab what you have and hold it against the cut. You can add fabric as the first compress becomes saturated, but don't pull the first piece off the wound, because it could remove the initial clot. You need to call 911 only if the bleeding doesn't stop within one to two minutes or you feel light-headed. "People get very freaked out by blood loss," says Dr. Mell, "but picture the mess from spilling a 12-ounce soda. We don't get worried until an adult has lost the equivalent of three to six cans."