You're playing your usual weekend pick-up basketball game with friends. Without warning, one of your teammates suddenly crumples to the ground. You scream out his name, but there's no response. His face turns pale and bluish, and you can't see his chest rise and fall to breathe. You listen for a heartbeat, but you can't find a pulse at all. You quickly grab your cell phone and dial 911.

Every year, this type of scenario is played out more than 600 times a day in the United States alone. Without rapid medical intervention, the prognosis is grim. Sudden cardiopulmonary arrest is the leading cause of death for all adults, male or female.

Fortunately, modern medicine has come up with a number of tools to combat cardiopulmonary arrest. Many of these emergency procedures require medical training and/or complex equipment, but one, cardiopulmonary resuscitation (CPR), can be used in the field by lay people with only a little bit of training. In this edition of How Stuff Works, we'll explore how this simple first aid technique can help save lives.

PLEASE NOTE: This article is not intended to be used as a method for teaching CPR. For proper CPR training, consult your local hospital or American Red Cross location for available classes.