Carl Zimmer investigates one of the biggest questions of all—What is life? It seems obvious, but are apples alive or is it only their trees? What if we find aliens and can't tell if they are alive or dead? These are not just philosophical questions—the debate in the US rages over the legal status of the life of a human zygote. When you are brain dead, are you really dead, or is that just a convenient description for the organ transplant surgeons? What life is matters to us emotionally, legally, and personally pretty much every day. But what is it exactly? Life's Edge is an utterly fascinating investigation that no one but one of the most celebrated science writers of our generation could craft.