What does it mean to be alive? Does life have a clear definition? On this week's episode, Carl Zimmer joins host Jim Underdown to discuss his new book, Life's Edge: The Search for What It Means to Be Alive to help answer those questions. Can we clearly define what it means to be alive? Scientists have been struggling with this question for centuries. For every rule or idea that's brought to the table, it seems a new species of plant or animal comes along that turns the whole thing on its head. For example, tardigrades, everyone's favorite microorganisms, are able to put themselves into a kind of suspended animation that stops their metabolization. Are they alive or dead at that point? Zimmer speaks about tardigrades and their special cryptobiosis, the intelligence of slime molds, and where viruses fit in the question of life. Carl Zimmer is an award-winning New York Times columnist and the author of fourteen books about science. His newest book is Life’s Edge: The Search for What It Means to Be Alive  (hardcover, Kindle, or audio.) You can find Zimmer on twitter @carlzimmer.