Since nearly every science curriculum in the country contains a section on living and non-living things, Bill Robertson believes that pretty much anyone who has taught the subject has run into difficulties. It seems as if no matter what criteria you use to distinguish between the two you can nearly always find exceptions. This article provides a few of the criteria for something to be living that teachers might run into in the average elementary science curriculum: (1) Living things require air; (2) Living things obtain and use energy; (3) Living things grow and reproduce; (4) Living things move; (5) Living things evolve; and (6) Living things excrete waste. Classifying things is an integral part of all science disciplines, so classifying things as living or nonliving is a useful exercise. Robertson suggests that the most effective way of dealing with the subject is to let students come up with their own list of criteria, adding and subtracting from the list as they examine more and more examples. He warns teachers to be prepared for the fact that, at some point, students are going to find examples that simply don't fit easily into either the living or nonliving camp. Even practicing scientists don't always agree on either the criteria or the classifications.