 The difficulty that Aristotle discusses is introduced by considering the way in which rain works. It rains because of material processes which can be specified as follows: when the warm air that has been drawn up is cooled off and becomes water, then this water comes down as rain (Phys. 198 b 19–21). It may happen that the corn in the field is nourished or the harvest is spoiled as a result of the rain, but it does not rain for the sake of any good or bad result. The good or bad result is just a coincidence (Phys. 198 b 21–23). So, why cannot all natural change work in the same way? For example, why cannot it be merely a coincidence that the front teeth grow sharp and suitable for tearing the food and the molars grow broad and useful for grinding the food (Phys. 198 b 23–27)? When the teeth grow in just this way, then the animal survives. When they do not, then the animal dies. More directly, and more explicitly, the way the teeth grow is not for the sake of the animal, and its survival or its death is just a coincidence (Phys. 198 b 29–32).