How do you define what things are living and dead? You might look at a sprinting cheetah and say it's clearly alive, whereas a chunk of rock is not -- but where do we draw the line? What might we expect extraterrestrials to look like, and would we even have the capacity to recognize them? And what does any of this have to do with Frankenstein, ancient Greek philosophers, or the possibility of finding a cell phone on Mars? Join Eagleman with guest Sara Walker, theoretical physicist at Arizona State University and author of the book “Life as No One Knows It”.