What adolescence is
Becoming sexually mature — that is, the arrival of youth — is called adolescence.
Through adolescence, the characteristics that allow males and females to be easily told apart arise. These are called secondary sexual characteristics: the characteristics that appear in males and in females along with adolescence.
Primary and secondary
The primary sexual characteristics are the reproductive organs themselves, which are present from birth. The secondary ones are everything else that changes at adolescence — body shape, hair, voice — and they appear only when the hormones arrive.
For the reproductive process to occur, reproductive cells — that is, gametes — must be formed. This takes place within the reproductive systems, and adolescence is when those systems begin to work.
Hormones do all of it
Every change on the next two pages is caused by a hormone: testosterone in males, oestrogen and progesterone in females. The changes are gradual, they begin at different ages in different people, and the age ranges given are typical rather than fixed.