The prime function of numbers, as a category, is the representation of magnitude. Each member denotes a different value. The category is ruled by a simple syntax. Once those rules are mastered there is no limit to the amount of numbers that can be correctly produced. Children will sometimes count for hours, or even days, after the fascinating system has first been revealed to them. Numbers are special. A person does not need much knowledge to list many, many numbers. In this respect other categories, such as birds, football players or psychological concepts, are much more difficult to learn.