Twenty-one is the first number to be the aliquot sum of three numbers 18, 51, This page contains text from Wikipedia, the Free Encyclopedia - https: Centuries are numbered ordinally in English and many other languages e. A centenary is a hundredth anniversary or a celebration of this, typically remembrance of an event which took place a hundred years earlier; and its adjectival form is centennial.
The 2nd century started at year , the 3rd at , etc. A century will only include one year, the centennial year, that starts with the century's number e. Also, there is no year 0 AD. Other centuries BCE follow the same pattern. The term girl may also be used to mean a young woman, and is often used as a synonym for daughter. Etymology The English word girl first appeared during the Middle Ages between and CE and came from the Anglo-Saxon words gerle also spelled girle or gurle.
The Anglo-Saxon word gerela meaning dress or clothing item also seems to have been used as a metonym in some sense. Girl has meant any young unmarried woman since about Its first noted meaning for sweetheart is The earliest known appearance of girl-friend is in and girl next door , meant as a teenaged female or young woman with a kind of wholesome appeal, dates only to Usage for adults The word girl is sometimes used to refer to an adult female, usually a younger one.
This usage may be considered derogatory or disrespectful in professional or other formal contexts, just as the term boy can be considered disparaging when applied to an adult man.
Hence, this usage is often deprecative. It can also be used deprecatively when used to discriminate against children "you're just a girl". Sexual reproduction involves the combining and mixing of genetic traits: Gametes can be identical in form and function known as isogamy , but in many cases an asymmetry has evolved such that two sex-specific types of gametes heterogametes exist known as anisogamy.
Among humans and other mammals , males typically carry XY chromosomes , whereas females typically carry XX chromosomes, which are a part of the XY sex-determination system. Other animals have a sex-determination system as well, such as the ZW sex-determination system in birds, and the X0 sex-determination system in insects.
The gametes produced by an organism are determined by its sex: Frequently, physical differences are associated with the different sexes of an organism; these sexual dimorphisms can reflect the different reproductive pressures the sexes experience.