Di fferent religions throughout the world claim to understand what happens to us after we die. Scientists are not as certain. They can explain, of course, what happens in and to our bodies at the moment of death and just after. To doctors, clinical death comes when the heart goes into cardiac arrest, which can occur from a variety of causes—from a car accident to illness. In effect, most of us die from cardiac arrest. The heart stops beating, cutting off the flow of blood, and thus oxygen, to the brain. Next comes biological death, as the brain, other organs, and cells stop functioning because of a lack of oxygen. Before reaching that point, however, in the window between clinical and biological death, doctors have been able to start the heart beating again, thus preventing death, or irreversible brain damage due to lack of oxygen. Thanks to research over the past several decades, doctors can now revive people whose hearts hav...