How do you know when someone is dead? It might sound like a question with an obvious answer, but the United States didn't have a legal definition of death until 1981. Science writer Dick Teresi says that this law actually made it easier to be declared dead than any other time in human history. Teresi has written a book that explores how modern medicine – including the process of organ donations - is blurring the line between life and death. He joins Doug on Thursday.