 For example, the set of natural numbers, denotable by {0, 1, 2, 3, 4, 5, ...}, has infinitely many elements, and we cannot use any normal number to give its size.Nonetheless, it turns out that infinite sets do have a well-defined notion of size (or more properly, of "cardinality", which is the technical term for the number of elements in a set), and not all infinite sets have the same cardinality.To understand what this means, we first examine what it "does not" mean.