It is said what aggregative properties are and also what emergent properties are, and examples are given each of kind of property. It is also explained why, even though all emergent properties are aggregative properties, not all aggregative properties are emergent properties. It is further made clear that, strictly speaking, emergence is a property of one's knowledge of a given kind of aggregate, and not of such aggregates themselves, this being why a property that is emergent at one time will, when additional information becomes available, cease to be emergent. And it is explained why, for this very reason, the right answer to the question 'why does X exist?' is never 'because X is an emergent property.'