By Joel Achenbach - republished from the pages of NATIONAL GEOGRAPHIC Magazine on science.nationalgeographic.com
Life has a way of being obvious—it literally scampers by, or growls, or curls up on the windowsill—and yet it's notoriously difficult to define in absolute terms. We say that life replicates. Life uses energy. Life adapts. Some forms of life have developed large central processing networks. In at least one instance, life has become profoundly self-aware.