It’s almost too easy to shred the logic of such lists. No one has ever managed to compile a set of physical properties that unites all living things and excludes everything we label inanimate. There are always exceptions. Most people do not consider crystals to be alive, for example, yet they are highly organized and they grow. Fire, too, consumes energy and gets bigger. In contrast, bacteria, tardigrades and even some crustaceans can enter long periods of dormancy during which they are not growing, metabolizing or changing at all, yet are not technically dead. How do we categorize a single leaf that has fallen from a tree? Most people would agree that, when attached to a tree, a leaf is alive: its many cells work tirelessly to turn sunlight, carbon dioxide and water into food, among other duties. When a leaf detaches from a tree, its cells do not instantly cease their activities. Does it die on the way to the ground; or when it hits the ground; or when all its individual cells finally expire? If you pluck a leaf from a plant and keep its cells nourished and happy inside a lab, is that life?