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One of the challenges for my students learning biology is summed up in one of my favorite sayings (that I’m sure some students are tired of hearing from me): “All the good concepts are fuzzy.” Take a basic concept like “living” versus “non-living,” for example. Obviously this is a fundamental concept for a biologist, since “biology” means the study of living things. Even here, though, we find that a precise definition of what is “alive” is a hard thing to nail down.